# `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 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))

### 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

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