# value initialization

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This is the initialization performed when a variable is constructed with an empty initializer.

### 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)

### Explanation

Value initialization is performed in these 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);
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);

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

In all cases, if the empty pair of braces `{}` is used and `T` is an aggregate type, 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++11)
1) if `T` is a class type with no default constructor or with a user-provided or deleted default constructor, the object is default-initialized; | (since C++11)

2) if `T` is a non-union class type without any user-provided constructors, every non-static data member and base-class component of T is value-initialized; | (until C++11)
2) if `T` is a class type with a default constructor that is neither user-provided nor deleted (that is, it may be a class with an implicitly-defined or defaulted default constructor), the object is zero-initialized and then it is default-initialized if it has a non-trivial default constructor; | (since C++11)

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

### Notes

A constructor is user-provided if it is user-declared and not explicitly defaulted on its first declaration.

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.

In C++98 prior to C++03 (which introduced value initialization), the expression new T() was classified as default initialization and specified zero-initialization.

References cannot be value-initialized.

As described in functional cast, the syntax T() (1) is prohibited for arrays, while T{} (5) is allowed.

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 default ctor, does not initialize i
    };

    struct B { A a; }; // implicitly-defined default 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++11, but for
    // a different reason: in post-DR1301 C++11, B{} is aggregate-initialization,
    // which then value-initializes A, which has a user-provided ctor)

### Example

Run this code

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

    struct T1
    {
        int mem1;
        std::string mem2;
    }; // implicit default constructor

    struct T2
    {
        int mem1;
        std::string mem2;
        T2(const T2&) { } // user-provided copy constructor
    };                    // no default constructor

    struct T3
    {
        int mem1;
        std::string mem2;
        T3() { } // user-provided default constructor
    };

    std::string s{}; // class => default-initialization, the value is ""

    int main()
    {
        int n{};                // scalar => zero-initialization, the value is 0
        double f = double();    // scalar => zero-initialization, the value is 0.0
        int* a = new int[10](); // array => value-initialization of each element
                                //          the value of each element is 0
        T1 t1{};                // class with implicit default constructor =>
                                //     t1.mem1 is zero-initialized, the value is 0
                                //     t1.mem2 is default-initialized, the value is ""
    //  T2 t2{};                // error: class with no default constructor
        T3 t3{};                // class with user-provided default constructor =>
                                //     t3.mem1 is default-initialized to indeterminate value
                                //     t3.mem2 is default-initialized, the value is ""
        std::vector<int> v(3);  // value-initialization of each element
                                // the value of each element is 0
        std::cout << s.size() << ' ' << n << ' ' << f << ' ' << a[9] << ' ' << v[2] << '\n';
        std::cout << t1.mem1 << ' ' << t3.mem1 << '\n';
        delete[] a;
    }

Possible output:

    0 0 0 0 0
    0 4199376

### 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 1301 | C++11 | defaulted default constructor skipped zero-init before construction | zero-init performed

### See also

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

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