static_assert declaration (since C++11)

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Performs compile-time assertion checking.


[edit] Syntax

static_assert( bool-constexpr , message ) (since C++11)
static_assert( bool-constexpr ) (since C++17)

[edit] 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)
message -
  • or an expression msg such that
    • msg.size() is implicitly convertible to std::size_t
    • is implicitly convertible to const char*
(since C++26)

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.

If message is not a string literal, it is evaluated only if bool-constexpr evaluates to false. The text of message is formed by the character sequence [ + msg.size()) which must denote a valid range, and each element in the sequence must be an integral constant expression.

(since C++26)

[edit] 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(bool-constexpr , "comment")
201411L (C++17) Single-argument static_assert(bool-constexpr )
202306L (C++26) user-generated static_assert messages

[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
                  "Swap requires copying");
    static_assert(std::is_nothrow_copy_constructible_v<T> &&
                  "Swap requires nothrow copy/assign");
    auto c = b;
    b = a;
    a = c;
template<class T>
struct data_structure
                  "Data structure requires default-constructible elements");
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)));
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 be
valid in a constant expression
CWG 2518 C++11 uninstantiated static_assert(false, ""); was error made well-formed

[edit] See also

shows the given error message and renders the program ill-formed
(preprocessing directive) [edit]
aborts the program if the user-specified condition is not true. May be disabled for release builds
(function macro) [edit]
conditionally removes a function overload or template specialization from overload resolution
(class template) [edit]
Type traits (C++11) defines a compile-time template-based interface to query or modify the properties of types
C documentation for Static assertion