static_assert declaration

 

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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.

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_copy_constructible<T>::value
               && std::is_nothrow_copy_assignable<T>::value,
                  "Swap requires nothrow copy/assign");
    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

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

See also

• #error
• assert
• type traits

C documentation for Static assertion