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consteval specifier (since C++20)

From cppreference.com
< cpp‎ | language
  • consteval - specifies that a function is an immediate function, that is, every call to the function must produce a compile-time constant

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[edit] Explanation

The consteval specifier declares a function or function template to be an immediate function, that is, every call to the function must (directly or indirectly) produce a compile time constant expression. It may not be applied to destructors, allocation functions, or deallocation functions. A consteval specifier implies inline. 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.

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

An invocation of an immediate function whose innermost non-block scope is not a function parameter scope of an immediate function must produce a constant expression; such an invocation (known as an immediate invocation) is always evaluated, even in an unevaluated operand.

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. 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

[edit] Keywords

consteval

[edit] Example

[edit] See also