Difference between revisions of "cpp/language/zero initialization"

Revision as of 19:51, 13 January 2015

Sets the initial value of an object to zero

Syntax

Explanation

Zero initialization is performed in the following situations:

The effects of zero initialization are:

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

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.

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
}

See also

• default initialization
• direct initialization
• value initialization
• copy initialization
• list initialization