# Static Assertion

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Performs compile-time assertion checking

### Syntax

`static_assert` `(` bool_constexpr `,` message `)` |  | (since C++11)
`static_assert` `(` bool_constexpr `)` |  | (since C++17)

### Explanation

bool_constexpr | - | a contextually converted constant expression of type bool
message | - | optional (since C++17)string literal that will appear as compiler error if bool_constexpr is false

A static assert declaration may appear at namespace and block scope (as a block declaration) and inside a class body (as a member declaration)

If bool_constexpr returns true, this declaration has no effect. Otherwise a compile-time error is issued, and the text of message, if any, is included in the diagnostic message.

message can be omitted. | (since C++17)

### Note

Since message has to be a string literal, it cannot contain dynamic information or even a constant expression that is not a string literal itself. In particular, it cannot contain the name of the template type argument.

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
CWG 2039 | C++11 | only the expression before conversion is required to be constant | the conversion must also be valid in a constant expression

### Example

Run this code

    #include <type_traits>

    template <class T>
    void swap(T& a, T& b)
    {
        static_assert(std::is_copy_constructible<T>::value,
                      "Swap requires copying");
        static_assert(std::is_nothrow_move_constructible<T>::value
                   && std::is_nothrow_move_assignable<T>::value,
                      "Swap may throw");
        auto c = b;
        b = a;
        a = c;
    }

    template <class T>
    struct data_structure
    {
        static_assert(std::is_default_constructible<T>::value,
                      "Data Structure requires default-constructible elements");
    };

    struct no_copy
    {
        no_copy ( const no_copy& ) = delete;
        no_copy () = default;
    };

    struct no_default
    {
        no_default () = delete;
    };

    int main()
    {
        int a, b;
        swap(a, b);

        no_copy nc_a, nc_b;
        swap(nc_a, nc_b); // 1

        data_structure<int> ds_ok;
        data_structure<no_default> ds_error; // 2
    }

Possible output:

    1: error: static assertion failed: Swap requires copying
    2: error: static assertion failed: Data Structure requires default-constructible elements

### See also

- #error
- assert
- type traits

C documentation for static_assert

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