# Value initialization

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This is the initialization performed when an object 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 object (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. | (since C++11)

The effects of value initialization are:

1) if `T` is a class type with no default constructor or with a user-declared (until C++11)user-provided or deleted (since C++11) default constructor, the object is default-initialized;
2) if `T` is a class type with a default constructor that is not user-declared (until C++11)neither user-provided nor deleted (since C++11) (that is, it may be a class with an implicitly-defined or defaulted default constructor), the object is zero-initialized and the semantic constraints for default-initialization are checked, and if `T` has a non-trivial default constructor, the object is default-initialized;
3) if `T` is an array type, each element of the array is value-initialized;
4) otherwise, the object is zero-initialized.

### 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.

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(), unless their allocator customizes the behavior of construct.

The standard specifies that zero-initialization is not performed when the class has a user-provided or deleted default constructor, which implies that whether said default constructor is selected by overload resolution is not considered. All known compilers performs additional zero-initialization if a non-deleted defaulted default constructor is selected.

    struct A
    {
        A() = default;

        template<class = void>
        A(int = 0) {} // A has a user-provided default constructor, which is not selected

        int x;
    };

    constexpr int test(A a)
    {
        return a.x; // the behavior is undefined if a's value is indeterminate
    }

    constexpr int zero = test(A());
    // ill-formed: the parameter is not zero-initialized according to the standard,
    // which results in undefined behavior that makes the program ill-formed in contexts
    // where constant evaluation is required.
    // However, such code is accepted by all known compilers.

    void f()
    {
        A a = A(); // not zero-initialized according to the standard
                   // but implementations generate code for zero-initialization nonetheless
    }

### Example

Run this code

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

    struct T1
    {
        int mem1;
        std::string mem2;
        virtual void foo() {} // make sure T1 is not an aggregate
    }; // 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
        assert(n == 0);
        double f = double();    // scalar => zero-initialization, the value is 0.0
        assert(f == 0.0);
        int* a = new int[10](); // array => value-initialization of each element
        assert(a[9] == 0);      //          the value of each element is 0
        T1 t1{};                // class with implicit default constructor =>
        assert(t1.mem1 == 0);   //     t1.mem1 is zero-initialized, the value is 0
        assert(t1.mem2 == "");  //     t1.mem2 is default-initialized, the value is ""
    //  T2 t2{};                // error: class with no default constructor
        T3 t3{};                // class with user-provided default constructor =>
        std::cout << t3.mem1;   //     t3.mem1 is default-initialized to indeterminate value
        assert(t3.mem2 == "");  //     t3.mem2 is default-initialized, the value is ""
        std::vector<int> v(3);  // value-initialization of each element
        assert(v[2] == 0);      // the value of each element is 0
        std::cout << '\n';
        delete[] a;
    }

Possible output:

    42

### 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 178 | C++98 | there was no value-initialization; empty initializer invoked default- initialization (though new T() also performs zero-initialization) | empty initializer invoke value-initialization
CWG 543 | C++98 | value-initialization for a class object without any user-provided constructors was equivalent to value- initializing each subobject (which need not zero- initialize a member with user-provided default constructor) | zero-initializes the entire object, then calls the default constructor
CWG 1301 | C++11 | value-initialization of unions with deleted default constructors led to zero-initialization | they are default-initialized
CWG 1368 | C++98 | any user-provided constructor caused zero-initialization to be skipped | only a user-provided default constructor skips zero-initialization
CWG 1502 | C++11 | value-initializing a union without a user-provided default constructor only zero-initialized the object, despite default member initializers | performs default- initialization after zero-initialization
CWG 1507 | C++98 | value-initialization for a class object without any user-provided constructors did not check the validity of the default constructor when the latter is trivial | the validity of trivial default constructor is checked

### See also

- default constructor
- `explicit`
- aggregate initialization
- list initialization

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