zero initialization

< cpp‎ | language

Sets the initial value of an object to zero


static T object ; (1)
T () ;

T t = {} ;
T {} ;


(since C++11)
char array [ n ] = ""; (3)


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 a character array 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.


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.


#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



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