Value-initialization - cppreference.com (original) (raw)

This is the initialization performed when an object 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);

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);

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.

[edit] 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.

[edit] Example

#include #include #include #include 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 int10; // 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 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:

[edit] Defect reports

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