Constant initialization

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


If a static or thread-local (since C++11) variable is constant-initialized (see below), constant initialization is performed instead of zero initialization before all other initializations.

A variable or temporary object obj is constant-initialized if.

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 (since C++11) object begins, and it may be performed at compile time.


The compiler is permitted to initialize other static and thread-local (since C++11) objects using constant initialization, if it can guarantee that the value would be the same as if the standard order of initialization was followed.

In practice, constant initialization is performed at compile time, and pre-calculated object representations are stored as part of the program image (e.g. in the .data section). If a variable is both const and constant initialized, its object representation may be stored in a read-only section of the program image (e.g. the .rodata section).


#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.

DR Applied to Behavior as published Correct behavior
CWG 441 C++98 references could not be constant initialized made constant initializable
CWG 1489 C++98 it was unclear whether value-initializing
an object can be a constant initialization
it can
CWG 1747 C++98 binding a reference to a function could not be constant initialization it can
CWG 1834 C++11 binding a reference to an xvalue could not be constant initialization it can
CWG 2026 C++98 zero-initialization was specified to always
occur first, even before constant initialization
no zero-initialization if
constant initialization applies
CWG 2366 C++98 default-initialization could not be constant
initialization (constant initializers were required)
it can

See also

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