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Showing content from https://en.cppreference.com/w/cpp/algorithm/../language/functions/function_contract_specifiers.html below:

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 pre attr (optional) ( expr ) (1) post attr (optional) ( result-name (optional) predicate ) (2) attr - any number of attributes result-name - identifier : identifier - name of a result binding of the associated function predicate - boolean expression that should evaluate to true

1) Precondition

2) Postcondition

[edit] Keywords

pre, post

[edit] Notes [edit] Example

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

#include <array>
#include <cmath>
#include <concepts>
#include <contracts>
#include <limits>
#include <print>
 
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))
{
    auto& [x, y, z]{vector};
    const auto norm{std::hypot(x, y, z)};
 
    x /= norm, y /= norm, z /= norm;
 
    return vector;
}
 
int main()
{
    const auto v = normalize<float>({0.3, 0.4, 0.5});
    std::println("{}", v);
 
    const auto w = normalize<float>({0, 0, 0}); // violates pre- and post- conditions
    std::println("{}", w);
}

Possible output:

[0.4242641, 0.56568545, 0.70710677]
[-nan, -nan, -nan]

[edit] References

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

9.(3+c ) Function contract specifiers [dcl.contract]

[edit] See also

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