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Value-initialization - cppreference.com

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

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);

when an object with dynamic storage duration is created by a

with the initializer consisting of an empty pair of parentheses

;

when a non-static data member or a base class is initialized using a

with an empty pair of parentheses

;

4) when a named object (automatic, static, or thread-local) is declared with the initializer consisting of a pair of braces.

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.

The effects of value-initialization are:

• If T is a (possibly cv-qualified) class type:

• If the default-initialization for T selects a constructor, and the constructor is not user-declared(until C++11)user-provided(since C++11), the object is first zero-initialized.
• In any case, the object is default-initialized.

• Otherwise, if T is an array type, each element of the array is value-initialized.
• Otherwise, the object is zero-initialized.

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 function-style cast, the syntax T() (1) is prohibited if T names an array type, 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.

#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:

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

• default constructor
• explicit
• aggregate initialization
• list-initialization

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