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Function contract specifiers (since C++26)

From cppreference.com

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.

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.

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.

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

Keywords

pre, post

Notes

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

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]

References

  • C++26 standard (ISO/IEC 14882:2026):
  • 9.(3+c ) Function contract specifiers [dcl.contract]

See also