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1.875-6.125-6.125z"></path></svg><div>Topics Covered</div></a></div><div class="markdown-body"><section class="main"><p><strong>Storage classes</strong> are type <a href="https://www.scaler.com/topics/what-is-access-specifiers-in-cpp/" target="_blank" title=""><strong>specifier in C++</strong></a> that helps specify the lifetime and visibility of variables and functions within a C++ program. C++ storage classes help determine the existence of a particular variable or function within a program's run time. They tell us which part of a program we can access a variable. Storage classes are especially significant because they determine how any variable may be used within a program or any method, especially in the context of their scope, lifetime, and storage.</p><p>Storage classes give us the following information about a variable or function:</p><ul> <li>The scope or the part of the program up to which the function or variable can be used.</li> <li>The location where the function or variable is stored.</li> <li>The initial value of variables.</li> <li>The lifetime of a variable.</li> <li>Accessibility of a variable.</li> </ul><p><strong>There are several storage classes in C++. They are as follows:</strong></p><ul> <li><span class="highlight--red">automatic</span></li> <li><span class="highlight--red">static</span></li> <li><span class="highlight--red">register</span></li> <li><span class="highlight--red">external</span></li> <li><span class="highlight--red">mutable</span></li> <li><span class="highlight--red">thread_local</span></li> </ul><p>Each of these storage classes implies different visibilities, lifetimes, and scopes of functions or variables that are declared with. The visibility or scope of a variable can be broadly divided into two categories:</p><ul> <li>Local</li> <li>Global</li> </ul><p><strong>Syntax</strong>:</p><p>The following syntax is used to define the storage class for a variable:</p><pre><div class="code-box_snippetContainer__cJ6zK"></div></pre><p>Where, <span class="highlight--red">storage_class</span> specifies the storage class. <span class="highlight--red">var_data_type</span> specifies the variable’s data type. <span class="highlight--red">var_name</span> specifies the variable name.</p><p><strong>The following keywords are used to specify the various storage classes:</strong></p> <div style="overflow:auto"><table><thead><tr><th style="text-align:center">Storage_Class</th><th style="text-align:center">Keyword</th></tr></thead><tbody><tr><td style="text-align:center">Automatic</td><td style="text-align:center"><span class="highlight--red">auto</span></td></tr><tr><td style="text-align:center">Static</td><td style="text-align:center"><span class="highlight--red">static</span></td></tr><tr><td style="text-align:center">Register</td><td style="text-align:center"><span class="highlight--red">register</span></td></tr><tr><td style="text-align:center">External</td><td style="text-align:center"><span class="highlight--red">extern</span></td></tr><tr><td style="text-align:center">Mutable</td><td style="text-align:center"><span class="highlight--red">mutable</span></td></tr><tr><td style="text-align:center">Thread Local</td><td style="text-align:center"><span class="highlight--red">thread_local</span></td></tr></tbody></table></div></section> <section class="main"><h2 id="types-of-storage-classes-in-c--" level="2">Types of Storage Classes in C++</h2><p>In the following section, we look at the types of storage classes in C++.</p> <div style="overflow:auto"><table><thead><tr><th style="text-align:center">Storage Class</th><th style="text-align:center">Keyword</th><th style="text-align:center">Lifetime</th><th style="text-align:center">Visibility</th><th style="text-align:center">Initial Value</th></tr></thead><tbody><tr><td style="text-align:center">Automatic</td><td style="text-align:center"><span class="highlight--red">auto</span></td><td style="text-align:center">Function Block</td><td style="text-align:center">Local</td><td style="text-align:center">Garbage</td></tr><tr><td style="text-align:center">External</td><td style="text-align:center"><span class="highlight--red">extern</span></td><td style="text-align:center">Whole Program</td><td style="text-align:center">Global</td><td style="text-align:center">Zero</td></tr><tr><td style="text-align:center">Static</td><td style="text-align:center"><span class="highlight--red">static</span></td><td style="text-align:center">Whole Program</td><td style="text-align:center">Local</td><td style="text-align:center">Zero</td></tr><tr><td style="text-align:center">Register</td><td style="text-align:center"><span class="highlight--red">register</span></td><td style="text-align:center">Function Block</td><td style="text-align:center">Local</td><td style="text-align:center">Garbage</td></tr><tr><td style="text-align:center">Mutable</td><td style="text-align:center"><span class="highlight--red">mutable</span></td><td style="text-align:center">Class</td><td style="text-align:center">Local</td><td style="text-align:center">Garbage</td></tr><tr><td style="text-align:center">Thread-Local</td><td style="text-align:center"><span class="highlight--red">thread_local</span></td><td style="text-align:center">Thread</td><td style="text-align:center">Local</td><td style="text-align:center"></td></tr></tbody></table></div><h3 id="1--automatic" level="3">1. Automatic</h3><h4>Description:</h4><p>The <strong>automatic storage class</strong> in C++ is the default storage class for all local variables. The <span class="highlight--red">auto</span> keyword is used to declare the automatic storage class for variables. The automatic storage class in C++ can also be used for the automatic deduction of data type and, as such, can be used while declaring a variable without specifying the data type.</p><p>Following C++11 standards, the auto keyword is now used for the automatic deduction of datatype in C++. The compiler automatically infers the data type of a variable declared auto based on the data assigned to it.</p><h4>Properties:</h4><p><strong>Automatic storage class in C++</strong> is the default storage class for variables declared without a specific storage class and has a local scope and lifetime.</p><p>The lifetime of an automatic variable is the function block within which the variable is declared, and the visibility of the variable is also local. Its initial value is a garbage value.</p><p><strong>Auto variables</strong> are declared without specifying the storage class for the variable as:</p><pre><div class="code-box_snippetContainer__cJ6zK"></div></pre><p>Where, <span class="highlight--red">var_data_type</span> specifies the variable’s data type. <span class="highlight--red">var_name</span> specifies the variable name.</p><h4>Example:</h4><pre><div class="code-box_snippetContainer__cJ6zK"></div></pre><p><strong>Output:</strong></p><pre><div class="code-box_snippetContainer__cJ6zK"></div></pre><p>In the code snippet above, we have declared <span class="highlight--red">x</span> inside the <span class="highlight--red">test()</span> function as a local variable, and it has been automatically set to the auto storage class. The variable <span class="highlight--red">y</span> has been declared with the auto keyword, which automatically infers its type using the assigned value of the variable. Within the <span class="highlight--red">main()</span> function, we have declared two more variables, x and y, similar to the other two. Due to the scopes of the variables being local, calling the <span class="highlight--red">test</span> function within <span class="highlight--red">main()</span> does not change the values of <span class="highlight--red">x</span> and <span class="highlight--red">y</span> in <span class="highlight--red">main()</span>.</p><h3 id="2--static" level="3">2. Static</h3><h4>Description:</h4><p>The <strong>static storage class</strong> in C++ is used to declare a variable that, once initialized, is not deallocated when the variable goes out of scope. The lifetime of such a variable is the entire program; as such, the variables do not lose their values within function calls. It is beneficial in recursive calls as static variables are initialized only once within a function block and can be changed on each recursive call without getting initialized over and over. Global variables may also be declared static, but such variables cannot be used outside the file in which it is declared.</p><h4>Properties:</h4><p><strong>Static storage class</strong> in C++ defines variables whose lifetime is equal to the program's lifetime, even if their scope is local.</p><p>The lifetime of a static variable is the entire program, but a static variable's visibility is local. Its initial value is zero.</p><p>Static variables are declared with the following <strong>syntax:</strong></p><pre><div class="code-box_snippetContainer__cJ6zK"></div></pre><p>Where, <span class="highlight--red">var_data_type</span> specifies the variable’s data type. <span class="highlight--red">var_name</span> specifies the variable name.</p><h4>Example:</h4><pre><div class="code-box_snippetContainer__cJ6zK"></div></pre><p><strong>Output</strong>:</p><pre><div class="code-box_snippetContainer__cJ6zK"></div></pre><p>In the code snippet above, we have declared a static variable, <span class="highlight--red">a</span>, within the <span class="highlight--red">test()</span> function. Within the <span class="highlight--red">main()</span> function, we have again declared a static variable, <span class="highlight--red">a</span>. When the <span class="highlight--red">test()</span> function is called within <span class="highlight--red">main()</span> twice, the local variable within <span class="highlight--red">test()</span> is initialized only once, and it retains its value within function calls. The <span class="highlight--red">a</span> variable declared within <span class="highlight--red">main()</span> is not changed with calls to <span class="highlight--red">test()</span> as it has a local scope.</p><h3 id="3--register" level="3">3. Register</h3><h4>Description:</h4><p>The <strong>register storage</strong> class in C++ utilizes CPU registers to store data to access data quickly in a function. The functionality of register variables is similar to automatic variables. The only difference is that the compiler tries to store the variable in the register instead of the memory if the registers are free. The compiler stores the register variables in the memory if no registers are free.</p><p>It is important to note that the address of a register variable cannot be accessed as they are stored in registers and not the memory. <strong>Register storage class</strong> in C++ is used to declare variables stored in registers by the compiler for faster access.</p><h4>Properties:</h4><p>Register variables are used when a program requires frequently accessing a variable. Registers, being a part of the processor, allow quick access to memory, much faster than standard memory. Register variables have local visibility, and their lifetime is the function block within which they are declared. Register variables are initialized with a garbage value.</p><p><strong>Syntax:</strong> Register variables are declared with the following syntax:</p><pre><div class="code-box_snippetContainer__cJ6zK"></div></pre><p>Where, <span class="highlight--red">var_data_type</span> specifies the variable’s data type. <span class="highlight--red">var_name</span> specifies the variable name.</p><h4>Example:</h4><pre><div class="code-box_snippetContainer__cJ6zK"></div></pre><p><strong>Output:</strong></p><pre><div class="code-box_snippetContainer__cJ6zK"></div></pre><p>In the code snippet above, we have declared a register variable in both <span class="highlight--red">test()</span> and <span class="highlight--red">main()</span>, initialized separately as the register has a local scope.</p><h3 id="4--external" level="3">4. External</h3><h4>Description:</h4><p>The <strong>external storage class</strong> in C++ is used when a variable or function is declared on a separate function block from the definition. That is, the variable or function may be defined in one code block or even file and then be declared and used in another code block or file. Using the extern keyword tells the compiler that the variable or function is defined elsewhere. It specifies external linkage for a variable. In the case of variable declarations without definition using the external storage class, no memory is allocated to the variable until it is defined elsewhere.</p><h4>Properties:</h4><p>The <strong>external storage class</strong> in C++ is used to declare variables defined elsewhere and have their lifetime and visibility as global.</p><p>The external storage class is specified using the <span class="highlight--red">extern</span> keyword. Its lifetime is the whole program, and its visibility is global, i.e., it is accessible by any block within the program. It is initialized with a value of 0. The external storage class is usually used when a variable or function is declared in separate files within a large program.</p><p><strong>Syntax:</strong> External variables are declared with the following syntax:</p><pre><div class="code-box_snippetContainer__cJ6zK"></div></pre><p>Where, <span class="highlight--red">var_data_type</span> specifies the variable’s data type. <span class="highlight--red">var_name</span> specifies the variable name.</p><h4>Example:</h4><pre><div class="code-box_snippetContainer__cJ6zK"></div></pre><p><strong>Output</strong></p><pre><div class="code-box_snippetContainer__cJ6zK"></div></pre><p>In the code snippet above, we have declared the variable <span class="highlight--red">x</span> as a global variable. The extern keyword within <span class="highlight--red">test()</span> specifies that the variable <span class="highlight--red">x</span> is declared somewhere else outside the current code block and, as such, allows us to access the global variable.</p><h3 id="5--thread_local" level="3">5. Thread_local</h3><h4>Description:</h4><p>The <strong>Thread-local</strong> storage class in C++ defines variables local to each thread created in the program. In the case of a program with multiple threads, each thread receives its instance of variables declared as thread-local. Thread_local variables may seem like static or global variables, but their lifetime is only the duration of a particular thread. That is, thread-local variables are created on thread creation and are disposed of when the thread is exited.</p><h4>Properties:</h4><p><strong>Thread Local storage class</strong> in C++ is used to declare variables local in scope and lifetime to the thread in which they are declared.</p><p>The Thread local storage class is specified using the <span class="highlight--red">thread_local</span> keyword. Its lifetime is the same as the lifetime of a thread, and its visibility is local.</p><p><strong>Syntax:</strong> Thread local variables are declared with the following syntax:</p><pre><div class="code-box_snippetContainer__cJ6zK"></div></pre><p>Where, <span class="highlight--red">var_data_type</span> specifies the variable’s data type. <span class="highlight--red">var_name</span> specifies the variable name.</p><h4>Example:</h4><pre><div class="code-box_snippetContainer__cJ6zK"></div></pre><p><strong>Output</strong></p><pre><div class="code-box_snippetContainer__cJ6zK"></div></pre><p>In the code snippet above, we have declared a thread-local variable with an initial value of zero. We have changed the thread-local <span class="highlight--red">val</span> value inside the test variable and printed it and its address. Inside the main, we have changed the value of <span class="highlight--red">val</span> and created a thread to run the <span class="highlight--red">test()</span> function. We have also printed the variable and its address before and after the thread. We observe that a separate instance of the <span class="highlight--red">val</span> is created in each thread with different initialization and address.</p><h3 id="6--mutable" level="3">6. Mutable</h3><h4>Description:</h4><p>The <strong>mutable storage class</strong> in C++ is used where any particular data member of a struct or a class must be modified in an otherwise <span class="highlight--red">const</span> object. Any constant object of a class or structure will always have constant data members and functions. But using the mutable storage class allows us to change those variables and use them as non-constant. It is used when one wishes to change only a few data members in a class or structure without changing the value of the others.</p><h4>Properties:</h4><p><strong>Mutable storage class</strong> in C++ is used to declare variables to be changed in an otherwise constant object or class.</p><p>The mutable storage class has a lifetime equal to that of the class, and it has local visibility. The initial value of a mutable storage class is a garbage value.</p><p><strong>Syntax:</strong> Mutable variables are declared with the following syntax:</p><pre><div class="code-box_snippetContainer__cJ6zK"></div></pre><p>Where, <span class="highlight--red">var_data_type</span> specifies the variable’s data type. <span class="highlight--red">var_name</span> specifies the variable name.</p><h4>Example:</h4><pre><div class="code-box_snippetContainer__cJ6zK"></div></pre><p><strong>Output</strong></p><pre><div class="code-box_snippetContainer__cJ6zK"></div></pre><p>In the code snippet above, we have declared a class Test with several data members and functions. Within <span class="highlight--red">main()</span>, we have declared constant objects of class <span class="highlight--red">Test</span>. Thus all data members of <span class="highlight--red">Test</span> are also constant. However, we have declared <span class="highlight--red">x</span> as mutable; hence, it can be modified.</p></section> <section class="summary"><h2 id="conclusion" level="2">Conclusion</h2><ol> <li>C++ storage classes help define the lifetime and visibility of variables and functions within a C++ program.</li> <li>The scope of C++ variables or functions can be either local or global.</li> <li>There are several C++ storage classes, namely <span class="highlight--red">Automatic, Register, Static, External, thread_local, and Mutable</span>.</li> <li>Storage Duration in C++ refers to the minimum time a variable exists within a program and might be usable or accessible.</li> <li>Linkage in C++ is the property that determines if a variable declared separately in multiple scopes refers to the same instance of the variable or separate.</li> </ol></section> <section class="main"><h2 id="related-topics:" level="2">Related Topics:</h2><ul> <li><a 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You will also get an overview of the visibility of variables and functions and their scopes.\nauthor: Santanu Baruah\ncategory: C++\n\n---\n\n:::section{.main}\n\n**Storage classes** are type [**specifier in C++**](https://www.scaler.com/topics/what-is-access-specifiers-in-cpp/) that helps specify the lifetime and visibility of variables and functions within a C++ program. C++ storage classes help determine the existence of a particular variable or function within a program's run time. They tell us which part of a program we can access a variable. Storage classes are especially significant because they determine how any variable may be used within a program or any method, especially in the context of their scope, lifetime, and storage.\n\n\nStorage classes give us the following information about a variable or function:\n\n\n- The scope or the part of the program up to which the function or variable can be used.\n- The location where the function or variable is stored.\n- The initial value of variables.\n- The lifetime of a variable.\n- Accessibility of a variable.\n\n**There are several storage classes in C++. They are as follows:**\n\n- `automatic`\n- `static`\n- `register`\n- `external`\n- `mutable`\n- `thread_local`\n\nEach of these storage classes implies different visibilities, lifetimes, and scopes of functions or variables that are declared with. The visibility or scope of a variable can be broadly divided into two categories:\n- Local\n- Global\n\n\n**Syntax**:\n\nThe following syntax is used to define the storage class for a variable:\n\n```cpp\nstorage_class var_data_type var_name;\n```\nWhere, \n`storage_class` specifies the storage class.\n`var_data_type` specifies the variable’s data type.\n`var_name` specifies the variable name.\n\n**The following keywords are used to specify the various storage classes:**\n\n\n| Storage_Class | Keyword |\n| :-------------: | :---------: | \n| Automatic | `auto` |\n| Static | `static` |\n| Register | `register`|\n| External | `extern` |\n| Mutable | `mutable` |\n| Thread Local | `thread_local` |\n\n:::\n\n:::section{.main}\n\n## Types of Storage Classes in C++ \n\nIn the following section, we look at the types of storage classes in C++.\n\n\n\n| Storage Class | Keyword | Lifetime | Visibility | Initial Value |\n| :-------------: | :-------: | :--------: |:------------:|:---------------:|\n| Automatic | `auto` | Function Block | Local | Garbage |\n| External | `extern` | Whole Program | Global | Zero |\n| Static | `static` | Whole Program | Local | Zero |\n| Register | `register` | Function Block | Local | Garbage |\n| Mutable | `mutable` | Class | Local | Garbage |\n| Thread-Local | `thread_local` | Thread | Local | |\n\n### 1. Automatic\n\n#### Description:\nThe **automatic storage class** in C++ is the default storage class for all local variables. The `auto` keyword is used to declare the automatic storage class for variables. The automatic storage class in C++ can also be used for the automatic deduction of data type and, as such, can be used while declaring a variable without specifying the data type. \n\nFollowing C++11 standards, the auto keyword is now used for the automatic deduction of datatype in C++. The compiler automatically infers the data type of a variable declared auto based on the data assigned to it. \n\n\n#### Properties:\n**Automatic storage class in C++** is the default storage class for variables declared without a specific storage class and has a local scope and lifetime.\n\nThe lifetime of an automatic variable is the function block within which the variable is declared, and the visibility of the variable is also local. Its initial value is a garbage value. \n\n**Auto variables** are declared without specifying the storage class for the variable as:\n```cpp\nvar_data_type var_name;\n```\nWhere, \n`var_data_type` specifies the variable’s data type.\n`var_name` specifies the variable name.\n\n\n#### Example:\n\n```cpp \n#include\u003ciostream\u003e\nusing namespace std;\n\nvoid test() {\n /* \n As of C++11, the auto keyword cannot be used to declare a variable with a data type. \n Thus we cannot have a declaration like 'auto int x=10;.'\n */\n\n // auto declaration of an int variable.\n int x = 10;\n\n // auto declaration without the datatype of the variable.\n auto y = 10;\n\n // adding the two variables.\n x = x + y;\n\n // Printing the two automatic variable; Output = 20 10.\n cout \u003c\u003c x \u003c\u003c \" \" \u003c\u003c y \u003c\u003c \"\\n\";\n}\n\nint main() {\n\n // Auto declaration of an int variable.\n int x = 0;\n\n // Auto declaration without datatype of the variable.\n auto y = \"Hello\";\n\n // Calling the test() function.\n test();\n\n // Modifying the x variable.\n x++;\n\n // Printing the two automatic variables; Output= 1 Hello.\n cout \u003c\u003c x \u003c\u003c \" \" \u003c\u003c y;\n}\n```\n**Output:**\n```plaintext\n10 10\n1 Hello\n```\nIn the code snippet above, we have declared `x` inside the `test()` function as a local variable, and it has been automatically set to the auto storage class. The variable `y` has been declared with the auto keyword, which automatically infers its type using the assigned value of the variable. Within the `main()` function, we have declared two more variables, x and y, similar to the other two. Due to the scopes of the variables being local, calling the `test` function within `main()` does not change the values of `x` and `y` in `main()`.\n\n### 2. Static\n\n\n#### Description:\nThe **static storage class** in C++ is used to declare a variable that, once initialized, is not deallocated when the variable goes out of scope. The lifetime of such a variable is the entire program; as such, the variables do not lose their values within function calls. It is beneficial in recursive calls as static variables are initialized only once within a function block and can be changed on each recursive call without getting initialized over and over. Global variables may also be declared static, but such variables cannot be used outside the file in which it is declared.\n\n#### Properties:\n**Static storage class** in C++ defines variables whose lifetime is equal to the program's lifetime, even if their scope is local.\n\nThe lifetime of a static variable is the entire program, but a static variable's visibility is local. Its initial value is zero. \n\nStatic variables are declared with the following **syntax:**\n```cpp\nstatic var_data_type var_name;\n```\nWhere, \n`var_data_type` specifies the variable’s data type.\n`var_name` specifies the variable name.\n\n\n#### Example:\n```cpp {\"output\":\"11 12 5\"}\n#include\u003ciostream\u003e\nusing namespace std;\n\nvoid test() {\n \n // Initializing a static variable.\n static int a = 10;\n \n // Updating the static variable.\n a++;\n \n // Printing the static variable.\n cout \u003c\u003c a \u003c\u003c \" \";\n}\n\nint main() {\n \n // Initializing another static variable.\n static int a = 5;\n \n // Calling the test() function; Output = 11.\n test();\n \n // Calling the test() function; Output = 12.\n test();\n \n // Printing the static variable; Output= 5.\n cout \u003c\u003c a;\n}\n```\n**Output**:\n```plaintext\n11 12 5\n```\nIn the code snippet above, we have declared a static variable, `a`, within the `test()` function. Within the `main()` function, we have again declared a static variable, `a`. When the `test()` function is called within `main()` twice, the local variable within `test()` is initialized only once, and it retains its value within function calls. The `a` variable declared within `main()` is not changed with calls to `test()` as it has a local scope.\n\n\n### 3. Register\n\n#### Description:\nThe **register storage** class in C++ utilizes CPU registers to store data to access data quickly in a function. The functionality of register variables is similar to automatic variables. The only difference is that the compiler tries to store the variable in the register instead of the memory if the registers are free. The compiler stores the register variables in the memory if no registers are free. \n\nIt is important to note that the address of a register variable cannot be accessed as they are stored in registers and not the memory. **Register storage class** in C++ is used to declare variables stored in registers by the compiler for faster access.\n\n#### Properties:\n\nRegister variables are used when a program requires frequently accessing a variable. Registers, being a part of the processor, allow quick access to memory, much faster than standard memory. Register variables have local visibility, and their lifetime is the function block within which they are declared. Register variables are initialized with a garbage value.\n\n**Syntax:**\nRegister variables are declared with the following syntax:\n```cpp\nregister var_data_type var_name;\n```\nWhere, \n`var_data_type` specifies the variable’s data type.\n`var_name` specifies the variable name.\n\n\n#### Example:\n\n```cpp {\"output\":\"12 0\"}\n#include\u003ciostream\u003e\nusing namespace std;\n\nvoid test() {\n // Declaration of a register variable.\n register int i=1;\n \n // Modifying the register variable.\n i = i + 11;\n \n // Printing the modified variable; Output= 12.\n cout \u003c\u003c i;\n}\n\nint main() {\n // Declaration of another register variable.\n register int i=0;\n \n // Calling test() function.\n test();\n \n // Printing the register variable; Output=0.\n cout\u003c\u003ci;\n \n}\n```\n**Output:**\n```plaintext\n12 0\n```\nIn the code snippet above, we have declared a register variable in both `test()` and `main()`, initialized separately as the register has a local scope. \n\n### 4. External\n\n#### Description:\nThe **external storage class** in C++ is used when a variable or function is declared on a separate function block from the definition. That is, the variable or function may be defined in one code block or even file and then be declared and used in another code block or file. Using the extern keyword tells the compiler that the variable or function is defined elsewhere. It specifies external linkage for a variable. In the case of variable declarations without definition using the external storage class, no memory is allocated to the variable until it is defined elsewhere.\n\n#### Properties:\n\nThe **external storage class** in C++ is used to declare variables defined elsewhere and have their lifetime and visibility as global.\n\nThe external storage class is specified using the `extern` keyword. Its lifetime is the whole program, and its visibility is global, i.e., it is accessible by any block within the program. It is initialized with a value of 0. The external storage class is usually used when a variable or function is declared in separate files within a large program. \n\n**Syntax:**\nExternal variables are declared with the following syntax:\n```cpp\nextern var_data_type var_name;\n```\nWhere, \n`var_data_type` specifies the variable’s data type.\n`var_name` specifies the variable name.\n\n\n#### Example:\n\n```cpp\n#include\u003ciostream\u003e\nusing namespace std;\n\n// Declaring the variable that is to be made extern.\nint x = 10;\n\nvoid test() {\n\n // Declaring an extern variable, i.e., it is defined elsewhere.\n extern int x;\n\n // Printing the extern variable; Output = 10.\n cout \u003c\u003c x;\n\n}\n\nint main() {\n\n // Calling the test function.\n test();\n\n return 0;\n}\n```\n**Output**\n```plaintext\n10\n```\nIn the code snippet above, we have declared the variable `x` as a global variable. The extern keyword within `test()` specifies that the variable `x` is declared somewhere else outside the current code block and, as such, allows us to access the global variable.\n \n### 5. Thread_local\n\n#### Description:\n\nThe **Thread-local** storage class in C++ defines variables local to each thread created in the program. In the case of a program with multiple threads, each thread receives its instance of variables declared as thread-local. Thread_local variables may seem like static or global variables, but their lifetime is only the duration of a particular thread. That is, thread-local variables are created on thread creation and are disposed of when the thread is exited.\n\n#### Properties:\n\n**Thread Local storage class** in C++ is used to declare variables local in scope and lifetime to the thread in which they are declared.\n\nThe Thread local storage class is specified using the `thread_local` keyword. Its lifetime is the same as the lifetime of a thread, and its visibility is local. \n\n**Syntax:**\nThread local variables are declared with the following syntax:\n```cpp\nthread_local var_data_type var_name;\n```\nWhere, \n`var_data_type` specifies the variable’s data type.\n`var_name` specifies the variable name.\n\n#### Example:\n```cpp {\"output\" : \"inside main(): 15 0x26a24f41d48\\n inside test(): 30 0x26a24f41fd8\\n inside main(): 15 0x26a24f41d48\"}\n#include \u003ciostream\u003e\n#include \u003cthread\u003e\nusing namespace std;\n\n// Declaring thread local variable.\nthread_local int val = 0;\n\nvoid test() {\n // Changing the value of the variable.\n val = 30;\n \n // Output: 30, address to its instance.\n \n cout \u003c\u003c \"inside test(): \" \u003c\u003c val \u003c\u003c \" \" \u003c\u003c \u0026val \u003c\u003c endl; \n}\n\nint main() {\n \n // Changing the value of the variable.\n val = 15;\n \n // Output: 15, address to val.\n cout \u003c\u003c \"inside main(): \" \u003c\u003c val \u003c\u003c \" \" \u003c\u003c \u0026val \u003c\u003c endl;\n \n // Calls test creates a different instance of val.\n thread t1(test);\n t1.join();\n \n // Output: 15, address to val.\n cout \u003c\u003c \"inside main(): \" \u003c\u003c val \u003c\u003c \" \" \u003c\u003c \u0026val \u003c\u003c endl;\n\n return 0;\n}\n```\n**Output**\n```plaintext\ninside main(): 15 0x26a24f41d48\ninside test(): 30 0x26a24f41fd8\ninside main(): 15 0x26a24f41d48\n```\nIn the code snippet above, we have declared a thread-local variable with an initial value of zero. We have changed the thread-local `val` value inside the test variable and printed it and its address. Inside the main, we have changed the value of `val` and created a thread to run the `test()` function. We have also printed the variable and its address before and after the thread. We observe that a separate instance of the `val` is created in each thread with different initialization and address. \n\n\n\n### 6. Mutable\n\n#### Description:\nThe **mutable storage class** in C++ is used where any particular data member of a struct or a class must be modified in an otherwise `const` object. Any constant object of a class or structure will always have constant data members and functions. But using the mutable storage class allows us to change those variables and use them as non-constant. It is used when one wishes to change only a few data members in a class or structure without changing the value of the others. \n\n#### Properties:\n**Mutable storage class** in C++ is used to declare variables to be changed in an otherwise constant object or class.\n\nThe mutable storage class has a lifetime equal to that of the class, and it has local visibility. The initial value of a mutable storage class is a garbage value. \n\n**Syntax:**\nMutable variables are declared with the following syntax:\n```cpp\nmutable var_data_type var_name;\n```\nWhere,\n`var_data_type` specifies the variable’s data type.\n`var_name` specifies the variable name.\n\n\n#### Example:\n\n```cpp {\"output\" : \"20\"}\n#include\u003ciostream\u003e\nusing namespace std;\n\nclass Test {\n \npublic:\n // Declaring a mutable variable.\n mutable int x;\n \n // Declaring a non-mutable variable.\n int y;\n \n Test() : x{},y{} {}\n};\n\nint main() {\n // Declaring a constant object of the class.\n const Test t;\n \n // Modifying the mutable variable and printing; Output =20.\n t.x = 20;\n cout \u003c\u003c t.x;\n \n // Uncommenting the following line throws an error as y is not mutable.\n // t.y = 10;\n // cout \u003c\u003c t.y;\n}\n```\n**Output**\n```plaintext\n20\n```\nIn the code snippet above, we have declared a class Test with several data members and functions. Within `main()`, we have declared constant objects of class `Test`. Thus all data members of `Test` are also constant. However, we have declared `x` as mutable; hence, it can be modified.\n:::\n\n:::section{.summary}\n\n## Conclusion \n\n1. C++ storage classes help define the lifetime and visibility of variables and functions within a C++ program.\n2. The scope of C++ variables or functions can be either local or global.\n3. There are several C++ storage classes, namely `Automatic, Register, Static, External, thread_local, and Mutable`.\n4. Storage Duration in C++ refers to the minimum time a variable exists within a program and might be usable or accessible.\n5. Linkage in C++ is the property that determines if a variable declared separately in multiple scopes refers to the same instance of the variable or separate. \n:::\n:::section{.main}\n## Related Topics:\n- [Data Types in C++](https://www.scaler.com/topics/cpp/data-types-in-cpp/)\n- [Nested Class in C++](https://www.scaler.com/topics/nested-class-in-cpp/)\n\n:::\n\n\n\n","course_video_data":[],"collaborators":{"authors":[{"name":"Santanu Baruah","slug":"0b6a2a221e10","email":"santanubaruah029@gmail.com","image":null,"linkedin_profile":null,"company":null,"designation":null,"validated_at":null,"topics_slug":"santanu-baruah","about":null,"github_profile":null,"twitter_profile":null,"headline":null}],"co_authors":[{"name":"Amisha Purswani","slug":"19ee1989b5db","email":"amishapurswani26@gmail.com","image":null,"linkedin_profile":"https://www.linkedin.com/in/amisha-purswani/","company":"I'm a Student","designation":"Frontend 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You will also get an overview of the visibility of variables and functions and their scopes.","author":"Santanu Baruah","category":"C++","publish_date":"undefined"},"contentData":"\n:::section{.main}\n\n**Storage classes** are type [**specifier in C++**](https://www.scaler.com/topics/what-is-access-specifiers-in-cpp/) that helps specify the lifetime and visibility of variables and functions within a C++ program. C++ storage classes help determine the existence of a particular variable or function within a program's run time. They tell us which part of a program we can access a variable. Storage classes are especially significant because they determine how any variable may be used within a program or any method, especially in the context of their scope, lifetime, and storage.\n\n\nStorage classes give us the following information about a variable or function:\n\n\n- The scope or the part of the program up to which the function or variable can be used.\n- The location where the function or variable is stored.\n- The initial value of variables.\n- The lifetime of a variable.\n- Accessibility of a variable.\n\n**There are several storage classes in C++. They are as follows:**\n\n- `automatic`\n- `static`\n- `register`\n- `external`\n- `mutable`\n- `thread_local`\n\nEach of these storage classes implies different visibilities, lifetimes, and scopes of functions or variables that are declared with. The visibility or scope of a variable can be broadly divided into two categories:\n- Local\n- Global\n\n\n**Syntax**:\n\nThe following syntax is used to define the storage class for a variable:\n\n```cpp\nstorage_class var_data_type var_name;\n```\nWhere, \n`storage_class` specifies the storage class.\n`var_data_type` specifies the variable’s data type.\n`var_name` specifies the variable name.\n\n**The following keywords are used to specify the various storage classes:**\n\n\n| Storage_Class | Keyword |\n| :-------------: | :---------: | \n| Automatic | `auto` |\n| Static | `static` |\n| Register | `register`|\n| External | `extern` |\n| Mutable | `mutable` |\n| Thread Local | `thread_local` |\n\n:::\n\n:::section{.main}\n\n## Types of Storage Classes in C++ \n\nIn the following section, we look at the types of storage classes in C++.\n\n\n\n| Storage Class | Keyword | Lifetime | Visibility | Initial Value |\n| :-------------: | :-------: | :--------: |:------------:|:---------------:|\n| Automatic | `auto` | Function Block | Local | Garbage |\n| External | `extern` | Whole Program | Global | Zero |\n| Static | `static` | Whole Program | Local | Zero |\n| Register | `register` | Function Block | Local | Garbage |\n| Mutable | `mutable` | Class | Local | Garbage |\n| Thread-Local | `thread_local` | Thread | Local | |\n\n### 1. Automatic\n\n#### Description:\nThe **automatic storage class** in C++ is the default storage class for all local variables. The `auto` keyword is used to declare the automatic storage class for variables. The automatic storage class in C++ can also be used for the automatic deduction of data type and, as such, can be used while declaring a variable without specifying the data type. \n\nFollowing C++11 standards, the auto keyword is now used for the automatic deduction of datatype in C++. The compiler automatically infers the data type of a variable declared auto based on the data assigned to it. \n\n\n#### Properties:\n**Automatic storage class in C++** is the default storage class for variables declared without a specific storage class and has a local scope and lifetime.\n\nThe lifetime of an automatic variable is the function block within which the variable is declared, and the visibility of the variable is also local. Its initial value is a garbage value. \n\n**Auto variables** are declared without specifying the storage class for the variable as:\n```cpp\nvar_data_type var_name;\n```\nWhere, \n`var_data_type` specifies the variable’s data type.\n`var_name` specifies the variable name.\n\n\n#### Example:\n\n```cpp \n#include\u003ciostream\u003e\nusing namespace std;\n\nvoid test() {\n /* \n As of C++11, the auto keyword cannot be used to declare a variable with a data type. \n Thus we cannot have a declaration like 'auto int x=10;.'\n */\n\n // auto declaration of an int variable.\n int x = 10;\n\n // auto declaration without the datatype of the variable.\n auto y = 10;\n\n // adding the two variables.\n x = x + y;\n\n // Printing the two automatic variable; Output = 20 10.\n cout \u003c\u003c x \u003c\u003c \" \" \u003c\u003c y \u003c\u003c \"\\n\";\n}\n\nint main() {\n\n // Auto declaration of an int variable.\n int x = 0;\n\n // Auto declaration without datatype of the variable.\n auto y = \"Hello\";\n\n // Calling the test() function.\n test();\n\n // Modifying the x variable.\n x++;\n\n // Printing the two automatic variables; Output= 1 Hello.\n cout \u003c\u003c x \u003c\u003c \" \" \u003c\u003c y;\n}\n```\n**Output:**\n```plaintext\n10 10\n1 Hello\n```\nIn the code snippet above, we have declared `x` inside the `test()` function as a local variable, and it has been automatically set to the auto storage class. The variable `y` has been declared with the auto keyword, which automatically infers its type using the assigned value of the variable. Within the `main()` function, we have declared two more variables, x and y, similar to the other two. Due to the scopes of the variables being local, calling the `test` function within `main()` does not change the values of `x` and `y` in `main()`.\n\n### 2. Static\n\n\n#### Description:\nThe **static storage class** in C++ is used to declare a variable that, once initialized, is not deallocated when the variable goes out of scope. The lifetime of such a variable is the entire program; as such, the variables do not lose their values within function calls. It is beneficial in recursive calls as static variables are initialized only once within a function block and can be changed on each recursive call without getting initialized over and over. Global variables may also be declared static, but such variables cannot be used outside the file in which it is declared.\n\n#### Properties:\n**Static storage class** in C++ defines variables whose lifetime is equal to the program's lifetime, even if their scope is local.\n\nThe lifetime of a static variable is the entire program, but a static variable's visibility is local. Its initial value is zero. \n\nStatic variables are declared with the following **syntax:**\n```cpp\nstatic var_data_type var_name;\n```\nWhere, \n`var_data_type` specifies the variable’s data type.\n`var_name` specifies the variable name.\n\n\n#### Example:\n```cpp {\"output\":\"11 12 5\"}\n#include\u003ciostream\u003e\nusing namespace std;\n\nvoid test() {\n \n // Initializing a static variable.\n static int a = 10;\n \n // Updating the static variable.\n a++;\n \n // Printing the static variable.\n cout \u003c\u003c a \u003c\u003c \" \";\n}\n\nint main() {\n \n // Initializing another static variable.\n static int a = 5;\n \n // Calling the test() function; Output = 11.\n test();\n \n // Calling the test() function; Output = 12.\n test();\n \n // Printing the static variable; Output= 5.\n cout \u003c\u003c a;\n}\n```\n**Output**:\n```plaintext\n11 12 5\n```\nIn the code snippet above, we have declared a static variable, `a`, within the `test()` function. Within the `main()` function, we have again declared a static variable, `a`. When the `test()` function is called within `main()` twice, the local variable within `test()` is initialized only once, and it retains its value within function calls. The `a` variable declared within `main()` is not changed with calls to `test()` as it has a local scope.\n\n\n### 3. Register\n\n#### Description:\nThe **register storage** class in C++ utilizes CPU registers to store data to access data quickly in a function. The functionality of register variables is similar to automatic variables. The only difference is that the compiler tries to store the variable in the register instead of the memory if the registers are free. The compiler stores the register variables in the memory if no registers are free. \n\nIt is important to note that the address of a register variable cannot be accessed as they are stored in registers and not the memory. **Register storage class** in C++ is used to declare variables stored in registers by the compiler for faster access.\n\n#### Properties:\n\nRegister variables are used when a program requires frequently accessing a variable. Registers, being a part of the processor, allow quick access to memory, much faster than standard memory. Register variables have local visibility, and their lifetime is the function block within which they are declared. Register variables are initialized with a garbage value.\n\n**Syntax:**\nRegister variables are declared with the following syntax:\n```cpp\nregister var_data_type var_name;\n```\nWhere, \n`var_data_type` specifies the variable’s data type.\n`var_name` specifies the variable name.\n\n\n#### Example:\n\n```cpp {\"output\":\"12 0\"}\n#include\u003ciostream\u003e\nusing namespace std;\n\nvoid test() {\n // Declaration of a register variable.\n register int i=1;\n \n // Modifying the register variable.\n i = i + 11;\n \n // Printing the modified variable; Output= 12.\n cout \u003c\u003c i;\n}\n\nint main() {\n // Declaration of another register variable.\n register int i=0;\n \n // Calling test() function.\n test();\n \n // Printing the register variable; Output=0.\n cout\u003c\u003ci;\n \n}\n```\n**Output:**\n```plaintext\n12 0\n```\nIn the code snippet above, we have declared a register variable in both `test()` and `main()`, initialized separately as the register has a local scope. \n\n### 4. External\n\n#### Description:\nThe **external storage class** in C++ is used when a variable or function is declared on a separate function block from the definition. That is, the variable or function may be defined in one code block or even file and then be declared and used in another code block or file. Using the extern keyword tells the compiler that the variable or function is defined elsewhere. It specifies external linkage for a variable. In the case of variable declarations without definition using the external storage class, no memory is allocated to the variable until it is defined elsewhere.\n\n#### Properties:\n\nThe **external storage class** in C++ is used to declare variables defined elsewhere and have their lifetime and visibility as global.\n\nThe external storage class is specified using the `extern` keyword. Its lifetime is the whole program, and its visibility is global, i.e., it is accessible by any block within the program. It is initialized with a value of 0. The external storage class is usually used when a variable or function is declared in separate files within a large program. \n\n**Syntax:**\nExternal variables are declared with the following syntax:\n```cpp\nextern var_data_type var_name;\n```\nWhere, \n`var_data_type` specifies the variable’s data type.\n`var_name` specifies the variable name.\n\n\n#### Example:\n\n```cpp\n#include\u003ciostream\u003e\nusing namespace std;\n\n// Declaring the variable that is to be made extern.\nint x = 10;\n\nvoid test() {\n\n // Declaring an extern variable, i.e., it is defined elsewhere.\n extern int x;\n\n // Printing the extern variable; Output = 10.\n cout \u003c\u003c x;\n\n}\n\nint main() {\n\n // Calling the test function.\n test();\n\n return 0;\n}\n```\n**Output**\n```plaintext\n10\n```\nIn the code snippet above, we have declared the variable `x` as a global variable. The extern keyword within `test()` specifies that the variable `x` is declared somewhere else outside the current code block and, as such, allows us to access the global variable.\n \n### 5. Thread_local\n\n#### Description:\n\nThe **Thread-local** storage class in C++ defines variables local to each thread created in the program. In the case of a program with multiple threads, each thread receives its instance of variables declared as thread-local. Thread_local variables may seem like static or global variables, but their lifetime is only the duration of a particular thread. That is, thread-local variables are created on thread creation and are disposed of when the thread is exited.\n\n#### Properties:\n\n**Thread Local storage class** in C++ is used to declare variables local in scope and lifetime to the thread in which they are declared.\n\nThe Thread local storage class is specified using the `thread_local` keyword. Its lifetime is the same as the lifetime of a thread, and its visibility is local. \n\n**Syntax:**\nThread local variables are declared with the following syntax:\n```cpp\nthread_local var_data_type var_name;\n```\nWhere, \n`var_data_type` specifies the variable’s data type.\n`var_name` specifies the variable name.\n\n#### Example:\n```cpp {\"output\" : \"inside main(): 15 0x26a24f41d48\\n inside test(): 30 0x26a24f41fd8\\n inside main(): 15 0x26a24f41d48\"}\n#include \u003ciostream\u003e\n#include \u003cthread\u003e\nusing namespace std;\n\n// Declaring thread local variable.\nthread_local int val = 0;\n\nvoid test() {\n // Changing the value of the variable.\n val = 30;\n \n // Output: 30, address to its instance.\n \n cout \u003c\u003c \"inside test(): \" \u003c\u003c val \u003c\u003c \" \" \u003c\u003c \u0026val \u003c\u003c endl; \n}\n\nint main() {\n \n // Changing the value of the variable.\n val = 15;\n \n // Output: 15, address to val.\n cout \u003c\u003c \"inside main(): \" \u003c\u003c val \u003c\u003c \" \" \u003c\u003c \u0026val \u003c\u003c endl;\n \n // Calls test creates a different instance of val.\n thread t1(test);\n t1.join();\n \n // Output: 15, address to val.\n cout \u003c\u003c \"inside main(): \" \u003c\u003c val \u003c\u003c \" \" \u003c\u003c \u0026val \u003c\u003c endl;\n\n return 0;\n}\n```\n**Output**\n```plaintext\ninside main(): 15 0x26a24f41d48\ninside test(): 30 0x26a24f41fd8\ninside main(): 15 0x26a24f41d48\n```\nIn the code snippet above, we have declared a thread-local variable with an initial value of zero. We have changed the thread-local `val` value inside the test variable and printed it and its address. Inside the main, we have changed the value of `val` and created a thread to run the `test()` function. We have also printed the variable and its address before and after the thread. We observe that a separate instance of the `val` is created in each thread with different initialization and address. \n\n\n\n### 6. Mutable\n\n#### Description:\nThe **mutable storage class** in C++ is used where any particular data member of a struct or a class must be modified in an otherwise `const` object. Any constant object of a class or structure will always have constant data members and functions. But using the mutable storage class allows us to change those variables and use them as non-constant. It is used when one wishes to change only a few data members in a class or structure without changing the value of the others. \n\n#### Properties:\n**Mutable storage class** in C++ is used to declare variables to be changed in an otherwise constant object or class.\n\nThe mutable storage class has a lifetime equal to that of the class, and it has local visibility. The initial value of a mutable storage class is a garbage value. \n\n**Syntax:**\nMutable variables are declared with the following syntax:\n```cpp\nmutable var_data_type var_name;\n```\nWhere,\n`var_data_type` specifies the variable’s data type.\n`var_name` specifies the variable name.\n\n\n#### Example:\n\n```cpp {\"output\" : \"20\"}\n#include\u003ciostream\u003e\nusing namespace std;\n\nclass Test {\n \npublic:\n // Declaring a mutable variable.\n mutable int x;\n \n // Declaring a non-mutable variable.\n int y;\n \n Test() : x{},y{} {}\n};\n\nint main() {\n // Declaring a constant object of the class.\n const Test t;\n \n // Modifying the mutable variable and printing; Output =20.\n t.x = 20;\n cout \u003c\u003c t.x;\n \n // Uncommenting the following line throws an error as y is not mutable.\n // t.y = 10;\n // cout \u003c\u003c t.y;\n}\n```\n**Output**\n```plaintext\n20\n```\nIn the code snippet above, we have declared a class Test with several data members and functions. Within `main()`, we have declared constant objects of class `Test`. Thus all data members of `Test` are also constant. However, we have declared `x` as mutable; hence, it can be modified.\n:::\n\n:::section{.summary}\n\n## Conclusion \n\n1. C++ storage classes help define the lifetime and visibility of variables and functions within a C++ program.\n2. The scope of C++ variables or functions can be either local or global.\n3. There are several C++ storage classes, namely `Automatic, Register, Static, External, thread_local, and Mutable`.\n4. Storage Duration in C++ refers to the minimum time a variable exists within a program and might be usable or accessible.\n5. Linkage in C++ is the property that determines if a variable declared separately in multiple scopes refers to the same instance of the variable or separate. \n:::\n:::section{.main}\n## Related Topics:\n- [Data Types in C++](https://www.scaler.com/topics/cpp/data-types-in-cpp/)\n- [Nested Class in C++](https://www.scaler.com/topics/nested-class-in-cpp/)\n\n:::\n\n\n\n","mediaList":[],"pageClass":"articlePage","rootClass":"articleLayout","subProduct":"article","fallback":{"https://www.scaler.com/topics/api/v1/search/courses/1/6/":[{"slug":"python-for-beginners","title":"Python Course for Beginners With Certification: Mastering the Essentials","category_type":"topics_course","cover_image_thumbnail":"https://www.scaler.com/topics/images/course_card_image_pybeg.webp","featured_image":"https://www.scaler.com/topics/images/course_featured_image_pybeg.webp","modules_count":16,"description":"Welcome to the free Python course with certificate for beginners, designed to help you kickstart your programming journey. 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