static_assert declaration (since C++11)

 

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Performs compile-time assertion checking.

Syntax

static_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.

Explanation

bool-constexpr	-	a contextually converted constant expression of type bool. Built-in conversions are not allowed, except for non-narrowing integral conversions to bool. (until C++23) an expression contextually converted to bool where the conversion is a constant expression (since C++23)
unevaluated-string	-	an unevaluated string literal that will appear as the error message
constant-expression	-	a constant expression msg satisfying all following conditions: msg.size() is implicitly convertible to std::size_t. msg.data() is implicitly convertible to const char*.

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:

• If the message matches the syntactic requirements of unevaluated-string, the text of the message is the text of the unevaluated-string.

Otherwise, given the following values: Let msg denote the value of constant-expression. Let len denote the value of msg.size(), which must be a converted constant expression of type std::size_t. Let ptr denote the expression msg.data(), implicitly converted to const char*. ptr must be a core constant expression. The text of the message is formed by the sequence of len code units, starting at ptr, of the ordinary literal encoding. For each integer i in [​0​, len), ptr[i] must be an integral constant expression.

(since C++26)

Notes

The standard does not require a compiler to print the verbatim text of message, though compilers generally do so as much as possible.

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.

(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)

Keywords

static_assert

Example

Run this code

#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

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
CWG 2518 (P2593R1)	C++11	uninstantiated static_assert(false, ""); was ill-formed	made well-formed

References

• C++23 standard (ISO/IEC 14882:2024):

• 9.1 Preamble [dcl.pre] (p: 10)

• C++20 standard (ISO/IEC 14882:2020):

• 9.1 Preamble [dcl.pre] (p: 6)

• C++17 standard (ISO/IEC 14882:2017):

• 10 Declarations [dcl.dcl] (p: 6)

• C++14 standard (ISO/IEC 14882:2014):

• 7 Declarations [dcl.dcl] (p: 4)

• C++11 standard (ISO/IEC 14882:2011):

• 7 Declarations [dcl.dcl] (p: 4)

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)
enable_if (C++11)	conditionally removes a function overload or template specialization from overload resolution (class template)
Type traits (C++11)	define compile-time template-based interfaces to query the properties of types
C documentation for Static assertion