value initialization(since C++03)
This is the initialization performed when a variable 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:
4) when a named variable (automatic, static, or thread-local) is declared with the initializer consisting of a pair of 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; 2) if T is a non-union class type without any user-provided constructors, every non-static data member and base-class component of T is value-initialized; | (until C++11) |
1) if T is a class type with no default constructor or with a user-provided or deleted default constructor, the object is default-initialized; 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 an implicitly-defined or defaulted default constructor), the object is zero-initialized and then it is default-initialized if it has a non-trivial default constructor; | (since C++11) |
T
is an array type, each element of the array is value-initialized;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()
.
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)
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;
}
Possible output:
0 0 0 0 0 0 4199376
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 |
See also
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http://en.cppreference.com/w/cpp/language/value_initialization