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 constant expression that is contextually convertible to 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 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.
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
#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
C documentation for static_assert |
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