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# value initialization

From cppreference.com
< cpp‎ | language

Provides the default initial value to a new object.

### [edit] Syntax

T`();` | (1) |
`new` T `();` | (2) |
Class`::`Class(...) `:` member`() {...` | (3) |
T object `{};` | (4) | (since C++11)
T`{};` | (5) | (since C++11)
`new` T `{};` | (6) | (since C++11)
Class`::`Class(...) `:`member`{} {...` | (7) | (since C++11)

### [edit] Explanation

Value initialization is performed in three situations:

1,5) when a nameless temporary object is created with the initializer consisting of an empty pair of parentheses or braces (since C++11).
2,6) when an object with dynamic storage duration is created by a new-expression with the initializer consisting of an empty pair of parentheses or braces (since C++11).

4) when a named variable (automatic, static, or thread-local) is declared with the initializer consisting of a pair of braces. | (since C++11)

3,7) when a non-static data member or a base class is initialized using a member initializer with an empty pair of parentheses or braces (since C++11)

In all cases, if the empty pair of braces `{`} is used and `T` is an aggregate type that is not a class type with a default constructor (until C++14), aggregate-initialization is performed instead of value-initialization.

If `T` is a class type that has no default constructor but has a constructor taking std::initializer_list, list-initialization is performed.

The effects of value initialization are:

1) If `T` is a class type with at least one user-provided constructor of any kind, the default constructor is called. | (until C++14)
1) If `T` is a class type with no default constructor or with a user-provided default constructor or with a deleted default constructor, the object is default-initialized. | (since C++14)

2) If `T` is an non-union class type without any user-provided constructors, then every non-static data member and base-class component of T is value-initialized | (until C++11)
2) If `T` is an non-union class type without any user-provided constructors, then the object is zero-initialized and then the implicitly-declared default constructor is called (unless it's trivial) | (since C++11) (until C++14)
2) If `T` is a class type without a user-provided or deleted default constructor (that is, it may be a class with a defaulted default constructor or with an implicitly-defined one) then the object is zero-initialized and then it is default-initialized if it has a non-trivial default constructor | (since C++14)

3) If `T` is an array type, each element of the array is value-initialized
4) Otherwise, the object is zero-initialized.

### [edit] Notes

The syntax T object(); does not initialize an object; it declares a function that takes no arguments and returns `T`. The way to value-initialize a named variable before C++11 was T object = T();, which value-initializes a temporary and then copy-initializes the object: most compilers optimize out the copy in this case.

References cannot be value-initialized.

All standard containers (std::vector, std::list, etc) value-initialize their elements when constructed with a single `size_type` argument or when grown by a call to resize().

Since C++11, value-initializing a class without a user-provided constructor, which has a member of a class type with a user-provided constructor zeroes out the member before calling its constructor:

    struct A {
        int i;
        A() {} // user-provided ctor, does not initialize i
    };

    struct B { A a; }; // no user-provided ctor

    std::cout << B().a.i << '\n'; // value-initializes a B temporary
                                  // leaves b.a.i uninitialized in C++03
                                  // sets b.a.i to zero in C++11
    // (note that B{}.a.i leaves b.a.i uninitialized in C++14, but for
    //  a different reason: C++14's B{} is aggregate-initialization)

### [edit] Example

Run this code

    #include <string>
    #include <vector>
    #include <iostream>

    struct T1 {
        int mem1;
        std::string mem2;
    }; // no constructors
    struct T2 {
        int mem1;
        std::string mem2;
        T2(const T2&) {} // a constructor, but no default
    };
    struct T3 {
        int mem1;
        std::string mem2;
        T3() {} // user-provided default ctor
    };

    std::string s{}; // calls default ctor, the value is "" (empty string)
    int main()
    {
        int n{};     // non-class value-initialization, value is 0
        double f = double(); // non-class value-init, value is 0.0
        int* a = new int[10](); // array of 10 zeroes

        T1 t1{}; // no ctors: zero-initialized
                 // t1.mem1 is zero-initialized
                 // t1.mem2 is default-initialized
    //    T2 t2{}; // error: has a ctor, but no default ctor
        T3 t3{}; // user-defined default ctor:
                 // t3.mem1 is default-initialized (the value is indeterminate)
                 // t3.mem2 is default-initialized

        std::vector<int> v(3); // value-initializes three ints

        std::cout << s.size() << ' ' << n << ' ' << f << ' ' << a[9] << ' ' << v[2] << '\n';
        std::cout << t1.mem1 << ' ' << t3.mem1 << '\n';
        delete[] a;
    }

Output:

    0 0 0 0 0
    0 4199376

### [edit] See also

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

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