Value-initialization

 

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Initialization

Initializer

Default initialization

Value initialization

Direct initialization

Copy-initialization

List initialization (C++11)

Aggregate initialization

Reference initialization

Copy elision

Static initialization

Zero initialization

Constant initialization

Dynamic non-local initialization

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Class member initialization

Member initializer list

Default member initializer (C++11)

 

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.

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:

• 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.

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 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.

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
CWG 2820	C++98	the default-initialization following the zero- initialization required a non-trivial constructor	not required
CWG 2859	C++98	value-initialization for a class object might involve zero-initialization even if the default-initialization does not actually select a user-provided constructor	there is no zero-initialization in this case

See also

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