Zero-initialization

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 `{}` `;` (since C++11)	(2)

CharT array `[` n `]` `=` `"` short-sequence `";`	(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 is initialized to the value obtained by explicitly converting the integer literal 0 (zero) to T.
If T is an non-union class type:

all padding bits are initialized to zero bits,
each non-static data member is zero-initialized,
each non-virtual base class subobject is zero-initialized, and
if the object is not a base class subobject, each virtual base class subobject is zero-initialized.

If T is a union type:

all padding bits are initialized to zero bits, and
the object’s first non-static named data member is zero-initialized.

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 694	C++98	zero-initialization for class types ignored padding	padding is initialized to zero bits
CWG 903	C++98	zero-initialization for scalar types set the initial value to the value converted from an integral constant expression with value 0	the object is initialized to the value converted from the integer literal 0
CWG 2026	C++98	zero-initialization was specified to always occur first, even before constant initialization	no zero-initialization if constant initialization applies
CWG 2196	C++98	zero-initialization for class types ignored base class subobjects	they are also zero-initialized
CWG 2253	C++98	it was unclear whether zero-initialization applies to unnamed bit-fields	it applies (all padding bits are initialized to zero bits)

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Initializer
Default initialization
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List initialization (C++11)
Aggregate initialization
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Copy elision
Static initialization
Zero initialization
Constant initialization
Dynamic non-local initialization
Ordered dynamic initialization
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Member initializer list
Default member initializer (C++11)