zero initialization

< cpp‎ | language
 
C++
 
C++ language
 
Initialization
 

Sets the initial value of an object to zero

Syntax

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 storage duration that is not subject to constant initialization (since C++14), 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 variables that aren't constant-initialized (since C++14) 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
std::string s; // zero-initialized to indeterminate value
               // then default-initialized to ""
int main(int argc, char* argv[])
{
    A a = A();
    std::cout << a.a << a.b << a.c << '\n';
    static int n = argc; // zero-initialized to 0
                         // then copy-initialized to argc
    delete p; // safe to delete a null pointer
}

Output: 

000

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