Difference between revisions of "cpp/language/value initialization"

Revision as of 23:46, 2 November 2021

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

Syntax

Explanation

Value initialization is performed in these situations:

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:

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.

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.

Since DR 543, 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)

The standard specifies that zero-initialization is not performed when the class has a user-provided or deleted default constructor, even if that default constructor is not selected by overload resolution. 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 <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, the value is 0
                            //     t1.mem2 is default-initialized, the value is ""
//  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);  // value-initialization of each element
                            // the value of each element is 0
    std::cout << s.size() << ' ' << n << ' ' << f << ' ' << a[9] << ' ' << v[2] << '\n';
    std::cout << t1.mem1 << ' ' << t3.mem1 << '\n';
    delete[] a;
}

0 0 0 0 0
0 4199376

Defect reports

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

See also

• default constructor
• explicit
• aggregate initialization
• list initialization