consteval_constinit.md

New Keywords - consteval and constinit

consteval

A function specified with the keyword consteval is an immediate function that is executed at compile-time.
Each call to this function must produce a compile-time constant expression.

This immediate function must satisfy all requirements applicable to constexpr functions.

C++14 - constexpr function	C++20 - consteval function
// executes at compile-time or run-time constexpr int squareNumX(int n) { return n * n; }	// executes at compile-time consteval int squareNumV(int n) { return n * n; }

Immediate functions (specified with consteval) cannot be applied to:

• destructors
• functions which allocate or deallocate

C++14	C++20
{ int x = 100; const int y = 100; int a = squareNumX(x); constexpr int b = squareNumX(x); // ERROR int c = squareNumX(y); constexpr int d = squareNumX(y); } // Error when x is not a constant expression but b is	{ int x = 100; const int y = 100; int a = squareNumV(x); // ERROR constexpr int b = squareNumV(x); // ERROR int c = squareNumV(y); constexpr int d = squareNumV(y); } // Error when the function argument (x) is not a constant expression

constinit

Applied to variables with static storage duration or thread storage duration.
constinit variables are initialized at compile-time.

constinit does not imply constness such as const or constexpr(2).

C++14 - constexpr	C++20 - constinit
// static storage duration constexpr int a = 100; int main() { ++a; // ERROR constexpr auto b = 100; } // Error since constexpr or const cannot be modifed // constexpr or const can be created locally	// static storage duration constinit int a = 100; int main() { ++a; constinit auto b = 100; // ERROR // b has thread storage duration constinit thread_local auto b = 100; } // Error since constinit cannot be created locally // constinit can be modified