Value-initialization
inline specifier noexcept specifier (C++11)typedef declaration This is the initialization performed when an object is constructed with an empty initializer.
[edit] Syntax
()
(1)
new T ()
(2)
::Class(...) : member () { ... }
(3)
{};
(4)
(since C++11)
{}
(5)
(since C++11)
new T {}
(6)
(since C++11)
::Class(...) : member {} { ... }
(7)
(since C++11)
[edit] 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.
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:
- If
Tis a (possibly cv-qualified) class type:
- If the default-initialization for
Tselects 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.
- If the default-initialization for
- Otherwise, if
Tis 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 <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
[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 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 |