constant initialization
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Sets the initial values of the static constants
Sintassi
static T & ref = constexpr;
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(1) | ||||||||
static T object = constexpr;
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(2) | ||||||||
Spiegazione
Constant initialization is performed after zero initialization of the static and thread-local objects and before all other initialization. Only the following variables are constant initialized:
constexpr and all constructor arguments (including implicit conversions) are constant expressions, and if the initializers in the constructor's initializer list and the brace-or-equal initializers of the class memebers only contain constant expressions.The effects of constant initialization are the same as the effects of the corresponding initialization, except that it's guaranteed that it is complete before any other initialization of a static or thread-local object begins, and it may be performed at compile time.
The object that was initialized by constant initialization can be used in constant expressions, e.g. in an array declaration.
Note
The compiler is permitted to initialize other static and thread-local objects using constant initialization, if it can guarantee that the value would be the same as if the standard order of initialization was followed. Such objects can be used in constant expressions, but this use is not portable.
Esempio
#include <iostream>
#include <array>
struct S {
static const int c;
};
const int d = 10 * S::c; // not a constant expression: S::c has no preceding
// initializer, this initialization happens after const
const int S::c = 5; // constant initialization, guaranteed to happen first
int main()
{
std::cout << "d = " << d << '\n';
std::array<int, S::c> a1; // OK: S::c is a constant expression
// std::array<int, d> a2; // error: d is not a constant expression
}
Output:
d = 50
