Performs compile-time assertion checking.
[edit] Syntaxstatic_assert( bool-constexpr , unevaluated-string ) (1) static_assert( bool-constexpr ) (2) (since C++17) static_assert( bool-constexpr , constant-expression ) (3) (since C++26)
Declares a static assertion. If the assertion fails, the program is ill-formed, and a diagnostic error message may be generated.
1) A static assertion with fixed error message.
2) A static assertion without error message.
3) A static assertion with user-generated error message.
This syntax can only be matched if syntax
(1)does not match.
[edit] Explanation bool-constexpr - unevaluated-string - an unevaluated string literal that will appear as the error message constant-expression - a constant expression msg satisfying all following conditions: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 is well-formed and evaluates to true, or is evaluated in the context of a template definition and the template is uninstantiated, this declaration has no effect. Otherwise a compile-time error is issued, and the user-provided message, if any, is included in the diagnostic message.
The text of the user-provided message is determined as follows:
The standard does not require a compiler to print the verbatim text of error message, though compilers generally do so as much as possible.
Since the error 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.
(until C++26) Feature-test macro Value Std Feature__cpp_static_assert 200410L (C++11) static_assert (syntax (1)) 201411L (C++17) Single-argument static_assert (syntax (2)) 202306L (C++26) User-generated error messages (syntax (3)) [edit] Keywords
[edit] Example
#include <format>
#include <type_traits>
static_assert(03301 == 1729); // since C++17 the message string is optional
template<class T>
void swap(T& a, T& b) noexcept
{
static_assert(std::is_copy_constructible_v<T>,
"Swap requires copying");
static_assert(std::is_nothrow_copy_constructible_v<T> &&
std::is_nothrow_copy_assignable_v<T>,
"Swap requires nothrow copy/assign");
auto c = b;
b = a;
a = c;
}
template<class T>
struct data_structure
{
static_assert(std::is_default_constructible_v<T>,
"Data structure requires default-constructible elements");
};
template<class>
constexpr bool dependent_false = false; // workaround before CWG2518/P2593R1
template<class T>
struct bad_type
{
static_assert(dependent_false<T>, "error on instantiation, workaround");
static_assert(false, "error on instantiation"); // OK because of CWG2518/P2593R1
};
struct no_copy
{
no_copy(const no_copy&) = delete;
no_copy() = default;
};
struct no_default
{
no_default() = delete;
};
#if __cpp_static_assert >= 202306L
// Not real C++ yet (std::format should be constexpr to work):
static_assert(sizeof(int) == 4, std::format("Expected 4, got {}", sizeof(int)));
#endif
int main()
{
int a, b;
swap(a, b);
no_copy nc_a, nc_b;
swap(nc_a, nc_b); // 1
[[maybe_unused]] data_structure<int> ds_ok;
[[maybe_unused]] 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 3: error: static assertion failed: Expected 4, got 2[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 becontract_assert statement (C++26) verifies an internal condition during execution[edit] conditionally removes a function overload or template specialization from overload resolution
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