Difference between revisions of "cpp/language/consteval"

Revision as of 09:58, 17 January 2022

• 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 (i.e. call out of an unevaluated context) to the function must (directly or indirectly) produce a compile time constant expression.

An immediate function is a constexpr function, and must satisfy the requirements applicable to constexpr functions or constexpr constructors, 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.

At most one of the constexpr, consteval, and constinit specifiers is allowed to appear within the same sequence of declaration specifiers. If any declaration of a function or function template contains a consteval specifier, then all declarations of that function or function template must contain that specifier.

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;
}
constexpr int 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
}
 
constexpr int dblsqr(int n) {
  return 2*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() { return 42; }
consteval auto g() { return &f; }
consteval int h(int (*p)() = g()) { return p(); }
constexpr int r = h();  // OK
constexpr auto e = g(); // ill-formed: a pointer to an immediate function is
                        // not a permitted result of a constant expression

Keywords

consteval

Example

#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.
constexpr unsigned factorial(unsigned n) {
    return n < 2 ? 1 : n * factorial(n - 1);
}
 
// With consteval we have a guarantee that the function will be evaluated at compile-time.
consteval unsigned combination(unsigned m, unsigned n) {
    // if !consteval { throw std::logic_error("Non-constexpr call to combination"); } // always OK in immediate functions
    return factorial(n)/factorial(m)/factorial(n-m);
}
 
static_assert(factorial(6) == 720);
static_assert(combination(4,8) == 70);
 
int main(int argc, const char*[]) {
 
    constexpr unsigned 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
}

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See also