Zero initialization

 

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Sets the initial value of an object to zero.

Syntax

Note that this is not the syntax for zero initialization, which does not have a dedicated syntax in the language. These are examples of other types of initializations, which might perform zero initialization.

static T object ;	(1)
T () ; T t = {} ; T {} ;	(2)	(since C++11)
CharT array [ n ] = "";	(3)

Explanation

Zero initialization is performed in the following situations:

1) For every named variable with static or thread-local (since C++11) storage duration that is not subject to constant initialization, before any other initialization.

2) As part of value-initialization sequence for non-class types and for members of value-initialized class types that have no constructors, including value initialization of elements of aggregates for which no initializers are provided.

3) When an array of any character type is initialized with a string literal that is too short, the remainder of the array is zero-initialized.

The effects of zero initialization are:

• If T is a scalar type, the object's initial value is the integral constant zero explicitly converted to T.

• If T is an non-union class type, all base classes and non-static data members are zero-initialized, and all padding is initialized to zero bits. The constructors, if any, are ignored.

• If T is a union type, the first non-static named data member is zero-initialized and all padding is initialized to zero bits.

• If T is array type, each element is zero-initialized.

• If T is reference type, nothing is done.

Notes

As described in non-local initialization, static and thread-local (since C++11) variables that aren't constant-initialized are zero-initialized before any other initialization takes place. If the definition of a non-class non-local variable has no initializer, then default initialization does nothing, leaving the result of the earlier zero-initialization unmodified.

A zero-initialized pointer is the null pointer value of its type, even if the value of the null pointer is not integral zero.

Example

Run this code

#include <string>
#include <iostream>
 
struct A
{
    int a, b, c;
};
 
double f[3];   // zero-initialized to three 0.0's
 
int* p;        // zero-initialized to null pointer value
               // (even if the value is not integral 0)
 
std::string s; // zero-initialized to indeterminate value, then
               // default-initialized to "" by the std::string default constructor
 
int main(int argc, char*[])
{
    delete p; // safe to delete a null pointer
 
    static int n = argc; // zero-initialized to 0 then copy-initialized to argc
    std::cout << "n = " << n << '\n';
 
    A a = A(); // the effect is same as: A a{}; or A a = {};
    std::cout << "a = {" << a.a << ' ' << a.b << ' ' << a.c << "}\n";
}

Possible output:

n = 1
a = {0 0 0}

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 277	C++98	pointers might be initialized with a non-constant expression of value 0, which is not a null pointer constant	must initialize with an integral constant expression of value 0
CWG 2026	C++98	zero initialization was specified to always occur first, even before constant initialization	no zero initialization if constant initialization applies

See also

• constructor
• copy assignment
• default constructor
• initialization
  • aggregate initialization
  • constant initialization
  • copy initialization
  • default initialization
  • direct initialization
  • list initialization
  • value initialization
• move assignment
• new