`contract_assert` statement (since C++26)

A contract_assert statement is a contract assertion that may appear in a function or lambda body to verify an internal condition. It ensures the condition holds during execution, triggering a violation (e.g. termination) in debug builds if the condition evaluates to false or the evaluation exits via an exception, and can be ignored in release builds for performance.

Syntax

contract_assert attr (optional) ( predicate ) ;

attr	-	any number of attributes
predicate	-	boolean expression that should evaluate to `true`

Keywords

contract_assert

Notes

Feature-test macro	Value	Std	Feature
`__cpp_contracts`	`202502L`	(C++26)	Contracts

Example

The contract_assert ensures that vector's norm is positive and either normal or subnormal.

template <std::floating_point T>
constexpr auto normalize(std::array<T, 3> vector) noexcept
    pre(/* is_normalizable(vector) */)
    post(/* vector: is_normalized(vector) */)
{
    auto& [x, y, z]{vector};
    const auto norm{std::hypot(x, y, z)};

    // debug check for normalization safety
    contract_assert(std::isfinite(norm) && norm > T(0));

    x /= norm, y /= norm, z /= norm;

    return vector;
}

Support status

C++26 feature	Paper(s)	GCC	Clang	MSVC	Apple Clang	EDG eccp	Intel C++	Nvidia HPC C++ (ex PGI)*	Nvidia nvcc	Cray
Contracts (FTM)*	P2900R14	16

References

C++26 standard (ISO/IEC 14882:2026):

8.(7+c ) Assertion statement [stmt.contract.assert]

assert	aborts the program if the user-specified condition is not `true`. May be disabled for release builds. (function macro)
Contract assertions (C++26)	specifies properties that must hold at certain points during execution
`static_assert` declaration (C++11)	performs compile-time assertion checking
function contract specifiers (C++26)	specifies preconditions (pre) and postconditions (post)
`[[assume(expression)]]` (C++23)	specifies that the expression will always evaluate to `true` at a given point (attribute specifier)

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contract_assert statement (since C++26)