zero initialization

< cpp‎ | language

Sets the initial value of an object to zero


[edit] Syntax

static T object ; (1)
int () ; (2)
char array [ n ] = ""; (3)

[edit] Explanation

Zero initialization is performed in the following situations:

1) For every named variable with static or thread-local storage duration, 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.
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 implicitly 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.

[edit] Notes

The static and thread-local variables are first zero-initialized and then initialized again as specified in the program, e.g. a function-local static is first zero-initialized at program startup, and then its constructor is called when the function is first entered. If the declaration of a non-class static 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.

[edit] Example

#include <string>
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[])
    static int n = argc; // zero-initialized to 0
                         // then copy-initialized to argc
    delete p; // safe to delete a null pointer

[edit] See also