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Value initialization

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

#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:

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.