static_assert declaration (since C++11)

 

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Declarators reference pointer array Block declarations simple-declaration → structured binding declaration (C++17) alias declaration(C++11) namespace alias definition using-declaration using-directive static_assert declaration (C++11) asm-declaration opaque enum declaration(C++11) Other declarations namespace definition function declaration class template declaration function template declaration explicit template instantiation(C++11) explicit template specialization linkage specification attribute declaration (C++11) empty declaration

 

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 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 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 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)

Notes

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.

Feature-test macro	Value	Std	Comment
__cpp_static_assert	200410L	(C++11)	static_assert(bool-constexpr , "comment")
	201411L	(C++17)	Single-argument static_assert(bool-constexpr )

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

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)	defines a compile-time template-based interface to query or modify the properties of types