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 |
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 |