contract_assert statement (since C++26)

 

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contract_assert (C++26)

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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;
}

References

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

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

See also

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)