Constant initialization

Sets the initial values of the static variables to a compile-time constant.

Explanation

Constant initialization is performed in the following cases:

The effects of constant initialization are the same as the effects of the corresponding initialization, except that it is guaranteed that it is complete before any other initialization of a staticor thread-local(since C++11)

Notes

The compiler is permitted to initialize other static and thread-local (since C++11)

Constant initialization usually happens when the program loads into memory, as part of initializing the program's runtime environment.

Example

#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
}

d = 50

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

See also

• constinit
• constexpr
• constructor
• converting constructor
• copy constructor
• default constructor
• explicit
• initialization
  • aggregate initialization
  • copy initialization
  • default initialization
  • direct initialization
  • list initialization
  • reference initialization
  • value initialization
  • zero initialization
• move constructor