constant initialization

 

C++

Language

Standard library headers

Concepts

Utilities library

Strings library

Containers library

Algorithms library

Iterators library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

Filesystem library (C++17)

Technical Specifications

 

C++ language

 

Initialization

Initializer

Default initialization

Value initialization

Direct initialization

Copy initialization

List initialization

Aggregate initialization

Reference initialization

Static initialization

Zero initialization

Constant initialization

Dynamic non-local initialization

Ordered dynamic initialization

Unordered dynamic initialization

Class member initialization

Member initializer list

in-class brace-or-equal initializer

 

Sets the initial values of the static constants

Syntax

static T & ref = constexpr;	(1)
static T object = constexpr;	(2)

Explanation

Constant initialization is performed after (until C++14)instead of (since C++14) zero initialization of the static and thread-local objects and before all other initialization. Only the following variables are constant initialized:

1) Static or thread-local references, if it is bound to static glvalue, to a temporary object (or its subobject), or to a function. and if every expression (including implicit conversions) in the initializer of the reference is a constant expression.

2) Static or thread-local object of class type that is initialized by a constructor call, if the constructor is 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 members only contain constant expressions.

3) Static or thread-local object (not necessarily of class type), that is not initialized by a constructor call, if the object is value-initialized or if every expression in its initializer is a constant expression.

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.

Notes

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.

Example

Run this code

#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

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 2026	C++14	zero-init was specified to always occur first, even before constant-init	no zero-init if constant init applies

See also

• constexpr specifier
• zero initialization