consteval specifier (since C++20)

 

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Overview Declaration syntax decl-specifier-seq Declarator Conflicting declarations Specifiers typedef inline virtual function specifier explicit function specifier friend constexpr (C++11) consteval (C++20) constinit (C++20) Storage class specifiers Translation-unit-local(C++20) class/struct union enum decltype (C++11) auto (C++11) alignas (C++11) constvolatile Pack indexing specifier(C++26)     Elaborated type specifier Attributes(C++11)

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• consteval - specifies that a function is an immediate function, that is, every call to the function must produce a compile-time constant

Explanation

The consteval specifier declares a function or function template to be an immediate function, that is, every potentially-evaluated call to the function must (directly or indirectly) produce a compile time constant expression.

An immediate function is a constexpr function, subject to its requirements as the case may be. Same as constexpr, a consteval specifier implies inline. However, it may not be applied to destructors, allocation functions, or deallocation functions.

A function or function template declaration specifying consteval may not also specify constexpr, and any redeclarations of that function or function template must also specify consteval.

A potentially-evaluated invocation of an immediate function whose innermost non-block scope is not a function parameter scope of an immediate function or the true-branch of a consteval if statement(since C++23) must produce a constant expression; such an invocation is known as an immediate invocation.

consteval int sqr(int n){return n*n;}constexprint r = sqr(100);// OK   int x =100;int r2 = sqr(x);// Error: Call does not produce a constant   consteval int sqrsqr(int n){return sqr(sqr(n));// Not a constant expression at this point, but OK}   constexprint dblsqr(int n){return2* sqr(n);// Error: Enclosing function is not consteval// and sqr(n) is not a constant}

An identifier expression that denotes an immediate function may only appear within a subexpression of an immediate invocation or within an immediate function context (i.e. a context mentioned above, in which a call to an immediate function needs not to be a constant expression). A pointer or reference to an immediate function can be taken but cannot escape constant expression evaluation:

consteval int f(){return42;} consteval auto g(){return&f;} consteval int h(int(*p)()= g()){return p();}constexprint r = h();// OKconstexprauto e = g();// ill-formed: a pointer to an immediate function is// not a permitted result of a constant expression

Notes

Feature-test macro	Value	Std	Feature
__cpp_consteval	201811L	(C++20)	Immediate functions
	202211L	(C++23) (DR20)	Making consteval propagate up

Keywords

consteval

Example

Run this code

#include <iostream>   // This function might be evaluated at compile-time, if the input// is known at compile-time. Otherwise, it is executed at run-time.constexprunsigned factorial(unsigned n){return n <2?1: n * factorial(n -1);}   // With consteval we enforce that the function will be evaluated at compile-time. consteval unsigned combination(unsigned m, unsigned n){return factorial(n)/ factorial(m)/ factorial(n - m);}   static_assert(factorial(6)==720); static_assert(combination(4, 8)==70);   int main(int argc, constchar*[]){constexprunsigned x{factorial(4)};std::cout<< x <<'\n';   [[maybe_unused]]unsigned y = factorial(argc);// OK// unsigned z = combination(argc, 7); // error: 'argc' is not a constant expression}

Output:

24

See also

constexpr specifier(C++11)	specifies that the value of a variable or function can be computed at compile time
constinit specifier(C++20)	asserts that a variable has static initialization, i.e. zero initialization and constant initialization
constant expression	defines an expression that can be evaluated at compile time