Function contract specifiers (since C++26)

Function contract specifiers (preconditions spelled with pre and postconditions spelled with post) are specifiers that may be applied to the declarator of a function or of a lambda expression to introduce a function contract assertion of the respective kind to the corresponding function.

They ensure the specified 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.

[edit] Precondition

A precondition (pre) is a predicate that the caller must ensure holds before invoking a function or lambda, checked in debug builds to validate inputs or state.

[edit] Postcondition

A postcondition (post) is a predicate that the callee must ensure holds after a function or lambda completes, verified in debug builds to confirm output or state.

[edit] Syntax

[edit] Keywords

pre, post

[edit] Example

• The precondition of function normalize requires caller to pass normalizable vector.
• The postcondition ensures that the function normalize returns a normalized vector.

template <std::floating_point T>
constexpr auto is_normalizable(const std::array<T, 3>& vector) noexcept
{
    const auto& [x, y, z] {vector};
    const auto norm{std::hypot(x, y, z)};
 
    return std::isfinite(norm) && norm > T {0};
}
 
template <std::floating_point T>
constexpr auto is_normalized(const std::array<T, 3>& vector) noexcept
{
    const auto& [x, y, z] {vector};
    const auto norm{std::hypot(x, y, z)};
    constexpr auto tolerance{010 * std::numeric_limits<T>::epsilon()};
 
    if (!is_normalizable(norm)) [[unlikely]]
        return false;
 
    return std::abs(norm - T{1}) <= tolerance;
}
 
template <std::floating_point T>
constexpr auto normalize(std::array<T, 3> vector) noexcept -> std::array<T, 3>
    pre(is_normalizable(vector))
    post(vector: is_normalized(vector));

[edit] References

[edit] See also