Performs compile-time assertion checking.
static_assert ( bool_constexpr , message ) | (since C++11) | |
static_assert ( bool_constexpr ) | (since C++17) |
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) |
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.
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 |
#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
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