static_assert declaration (since C++11)

 

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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 (until C++23) an expression contextually converted to bool where the conversion is a constant expression (since C++23)
message	-	optional (since C++17) string literal that will appear as compiler error if bool-constexpr is false, except that characters not in the basic source character set are not required to appear (until C++23).

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)

Built-in conversions are not allowed, except for non-narrowing integral conversions to bool.

(until C++23)

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>
 
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");
};
 
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
 
    [[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

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	shows the given error message and renders the program ill-formed (preprocessing directive)
assert	aborts the program if the user-specified condition is not true. May be disabled for release builds (function macro)
abort	causes abnormal program termination (without cleaning up) (function)
exit	causes normal program termination with cleaning up (function)
enable_if (C++11)	conditionally removes a function overload or template specialization from overload resolution (class template)
Type traits (C++11)	defines a compile-time template-based interface to query or modify the properties of types