Value-initialization
This is the initialization performed when an object is constructed with an empty initializer.
# 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
#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;
}
# Defect reports
| 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 invokevalue-initialization |
| CWG 543 | C++98 | value-initialization for a class object without anyuser-provided constructors was equivalent to value-initializing each subobject (which need not zero-initialize a member with user-provided default constructor) | zero-initializesthe entire object,then calls thedefault constructor |
| CWG 1301 | C++11 | value-initialization of unions with deleteddefault constructors led to zero-initialization | they aredefault-initialized |
| CWG 1368 | C++98 | any user-provided constructor causedzero-initialization to be skipped | only a user-provideddefault constructorskips zero-initialization |
| CWG 1502 | C++11 | value-initializing a union without a user-provideddefault constructor only zero-initialized theobject, despite default member initializers | performs default-initialization afterzero-initialization |
| CWG 1507 | C++98 | value-initialization for a class object without anyuser-provided constructors did not check the validityof the default constructor when the latter is trivial | the validity of trivialdefault constructoris 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 involvezero-initialization even if the default-initializationdoes not actually select a user-provided constructor | there is nozero-initializationin this case |