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The keyword `constexpr` was introduced in C++11 and improved in C++14. It means *constant expression*. Like `const`, it can be applied to variables so that a compiler error will be raised if any code attempts to modify the value. Unlike `const`, `constexpr` can also be applied to functions and class constructors. `constexpr` indicates that the value, or return value, is constant and, if possible, will be computed at compile time. A `constexpr` integral value can be used wherever a const integer is required, such as in template arguments and array declarations. And when a value can be computed at compile time instead of run time, it can help your program run faster and use less memory.
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## Syntax
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```
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The keyword **constexpr** was introduced in C++11 and improved in C++14. It means *constant expression*. Like **const**, it can be applied to variables so that a compiler error will be raised if any code attempts to modify the value. Unlike **const**, **constexpr** can also be applied to functions and class constructors. **constexpr** indicates that the value, or return value, is constant and, if possible, will be computed at compile time.
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A **constexpr** integral value can be used wherever a const integer is required, such as in template arguments and array declarations. And when a value can be computed at compile time instead of run time, it can help your program run faster and use less memory.
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To limit the complexity of computing compile time constants, and their potential impacts of compilation time, the C++14 standard requires that the types involved in constant expressions be restricted to [literal types](trivial-standard-layout-and-pod-types.md#literal_types).
One or more parameters which must be a literal type (as listed below) and must itself be a constant expression.
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## Return Value
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A constexpr variable or function must return one of the literal types, as listed below.
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## Literal types
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To limit the complexity of computing compile time constants, and their potential impacts of compilation time, the C++14 standard requires that the types involved in constant expressions be restricted to literal types. A literal type is one whose layout can be determined at compile time. The following are the literal types:
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1. void
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2. scalar types
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3. references
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4. Arrays of void, scalar types or references
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5. A class that has a trivial destructor, and one or more constexpr constructors that are not move or copy constructors. Additionally, all its non-static data members and base classes must be literal types and not volatile.
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## constexpr variables
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The primary difference between const and constexpr variables is that the initialization of a const variable can be deferred until run time whereas a constexpr variable must be initialized at compile time. All constexpr variables are const.
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- A variable can be declared with `constexpr`, if it has a literal type and is initialized. If the initialization is performed by a constructor, the constructor must be declared as `constexpr`.
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- A reference may be declared as constexpr if the object that it references has been initialized by a constant expression and any implicit conversions that are invoked during initialization are also constant expressions.
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- All declarations of a `constexpr` variable or function must have the `constexpr` specifier.
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```cpp
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constexprfloat x = 42.0;
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constexprfloat y{108};
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constexpr float z = exp(5, 3);
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constexpr int i; // Error! Not initialized
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int j = 0;
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constexpr int k = j + 1; //Error! j not a constant expression
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```
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## constexpr functions
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A `constexpr` function is one whose return value can be computed at compile when consuming code requires it. When its arguments are `constexpr` values, and consuming code requires the return value at compile time, for example to initialize a `constexpr` variable or provide a non-type template argument, it produces a compile-time constant. When called with non-`constexpr` arguments, or when its value is not required at compile-time, it produces a value at run time like a regular function. (This dual behavior saves you from having to write `constexpr` and non-`constexpr` versions of the same function.)
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A `constexpr` function or constructor is implicitly `inline`.
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The following rules apply to constexpr functions:
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constexpr ctor (params);
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```
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## Parameters
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*params*
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One or more parameters which must be a literal type and must itself be a constant expression.
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## Return Value
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A constexpr variable or function must return a [literal type](trivial-standard-layout-and-pod-types.md#literal_types).
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## constexpr variables
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The primary difference between const and constexpr variables is that the initialization of a const variable can be deferred until run time whereas a constexpr variable must be initialized at compile time. All constexpr variables are const.
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- A variable can be declared with **constexpr**, if it has a literal type and is initialized. If the initialization is performed by a constructor, the constructor must be declared as **constexpr**.
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- A reference may be declared as constexpr if the object that it references has been initialized by a constant expression and any implicit conversions that are invoked during initialization are also constant expressions.
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- All declarations of a **constexpr** variable or function must have the **constexpr** specifier.
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```cpp
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constexprfloat x = 42.0;
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constexprfloat y{108};
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constexpr float z = exp(5, 3);
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constexpr int i; // Error! Not initialized
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int j = 0;
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constexpr int k = j + 1; //Error! j not a constant expression
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```
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## constexpr functions
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A **constexpr** function is one whose return value can be computed at compile when consuming code requires it. When its arguments are **constexpr** values, and consuming code requires the return value at compile time, for example to initialize a **constexpr** variable or provide a non-type template argument, it produces a compile-time constant. When called with non-`constexpr** arguments, or when its value is not required at compile-time, it produces a value at run time like a regular function. (This dual behavior saves you from having to write **constexpr** and non-`constexpr** versions of the same function.)
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A **constexpr** function or constructor is implicitly **inline**.
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The following rules apply to constexpr functions:
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- A `constexpr` function must accept and return only literal types.
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- A **constexpr** function must accept and return only [literal types](trivial-standard-layout-and-pod-types.md#literal_types).
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- A `constexpr` function can be recursive.
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- A **constexpr** function can be recursive.
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- It cannot be [virtual](../cpp/virtual-cpp.md). A a constructor cannot be defined as constexpr if the enclosing class has any virtual base classes.
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- The body can be defined as `= default` or `= delete`.
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- The body can be defined as **= default** or **= delete**.
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- The body can contain no `goto` statements or try blocks.
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- The body can contain no **goto** statements or try blocks.
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- An explicit specialization of a non-constexpr template can be declared as `constexpr`:
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- An explicit specialization of a `constexpr` template does not have to also be `constexpr`:
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- An explicit specialization of a non-constexpr template can be declared as **constexpr`:
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- An explicit specialization of a **constexpr** template does not have to also be **constexpr`:
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<!--conformance note-->
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The following rules apply to constexpr functions in Visual Studio 2017 and later:
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The following rules apply to **constexpr** functions in Visual Studio 2017 and later:
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- It may contain if and switch statements, and all looping statements including for, range-based for, while, and do-while
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- It may contain **if** and **switch** statements, and all looping statements including **for**, range-based for, **while**, and **do-while**.
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- It may contain local variable declarations, but the variable must be initialized, must be a literal type, and cannot be static or thread-local. The locally-declared variable is not required to be const and may mutate.
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- A constexpr non-static member function is not required to be implicitly const.
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```cpp
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constexpr float exp(float x, int n)
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{
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return n == 0 ? 1 :
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n % 2 == 0 ? exp(x * x, n / 2) :
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exp(x * x, (n - 1) / 2) * x;
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};
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```
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```cpp
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constexpr float exp(float x, int n)
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{
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return n == 0 ? 1 :
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n % 2 == 0 ? exp(x * x, n / 2) :
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exp(x * x, (n - 1) / 2) * x;
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};
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```
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> [!TIP]
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> Note: In the Visual Studio debugger, you can tell whether a `constexpr` function is being evaluated at compile time by putting a breakpoint inside it. If the breakpoint is hit, the function was called at run-time. If not, then the function was called at compile time.
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## Example
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The following example shows `constexpr` variables, functions and a user-defined type. Note that in the last statement in main(), the `constexpr` member function GetValue() is a run-time call because the value is not required to be known at compile time.
cout << "The value of foo is " << foo.GetValue() << endl;
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}
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```
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## Requirements
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Visual Studio 2015
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## See Also
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[Declarations and Definitions](../cpp/declarations-and-definitions-cpp.md)
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[const](../cpp/const-cpp.md)
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> Note: In the Visual Studio debugger, when debugging a non-optimised Debug build, you can tell whether a **constexpr** function is being evaluated at compile time by putting a breakpoint inside it. If the breakpoint is hit, the function was called at run-time. If not, then the function was called at compile time.
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## Example
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The following example shows **constexpr** variables, functions and a user-defined type. Note that in the last statement in main(), the **constexpr** member function GetValue() is a run-time call because the value is not required to be known at compile time.
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