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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) |
T `()` `;` T t `=` `{}` `;` T `{}` `;` | (2) | (since C++11)
`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 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.

### [edit] 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.

### [edit] Example

Run this code

    #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
    }

Output:

    000

### [edit] 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

### [edit] See also

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

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