value initialization

< cpp‎ | language

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


[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 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).
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, 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 default constructor or with a deleted default constructor, the object is default-initialized.
(since C++11)
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 a class type with a default constructor that is neither user-provided nor deleted (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++11)
3) If T is an array type, each element of the array is value-initialized
4) Otherwise, the object is zero-initialized.

[edit] 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.

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)

[edit] Example

#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
                            //     t1.mem2 is default-initialized
//  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);  // the value of each element is value-initalized to 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

[edit] 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

[edit] See also