Function contract specifiers (since C++26)

 

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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

preattr (optional)(expr)	(1)
postattr (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.

Run this code

#include <array>#include <cmath>#include <concepts>#include <contracts>#include <limits>#include <print>   template<std::floating_point T>constexprauto is_normalizable(conststd::array<T, 3>& vector)noexcept{constauto&[x, y, z]{vector};constauto norm{std::hypot(x, y, z)};   returnstd::isfinite(norm)&& norm > T {0};}   template<std::floating_point T>constexprauto is_normalized(conststd::array<T, 3>& vector)noexcept{constauto&[x, y, z]{vector};constauto norm{std::hypot(x, y, z)};constexprauto tolerance{010*std::numeric_limits<T>::epsilon()};   if(!is_normalizable(norm))[[unlikely]]returnfalse;   return std::abs(norm - T{1})<= tolerance;}   template<std::floating_point T>constexprauto 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};constauto norm{std::hypot(x, y, z)};   x /= norm, y /= norm, z /= norm;   return vector;}   int main(){constauto v = normalize<float>({0.3, 0.4, 0.5});std::println("{}", v);   constauto w = normalize<float>({0, 0, 0});// violates pre- and post- conditionsstd::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

Contract assertions(C++26)	specifies properties that must hold at certain points during execution
contract_assert statement(C++26)	verifies an internal condition during execution