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# zero initialization

From cppreference.com
< 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

Run this code

    #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

- default initialization
- direct initialization
- value initialization
- copy initialization
- list initialization

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