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

Revision as of 13:51, 3 May 2020

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 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

#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
}

000

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

See also

• copy elision
• converting constructor
• copy assignment
• copy constructor
• default constructor
• destructor
• explicit
• initialization
  • aggregate initialization
  • constant initialization
  • copy initialization
  • default initialization
  • direct initialization
  • initializer list
  • list initialization
  • reference initialization
  • value initialization
• move assignment
• move constructor
• new