Static Assertion

< cpp‎ | language

Performs compile-time assertion checking


[edit] Syntax

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

[edit] 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)

[edit] 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.

[edit] 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

[edit] Example

#include <type_traits>
template <class T>
void swap(T& a, T& b)
                  "Swap requires copying");
               && std::is_nothrow_move_assignable<T>::value,
                  "Swap may throw");
    auto c = b;
    b = a;
    a = c;
template <class T>
struct data_structure
                  "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

[edit] See also

C documentation for static_assert