Initializes an object from explicit set of constructor arguments.
Syntax
T object ( arg ); T object | (1) | |
T object { arg }; T object | (2) | (since C++11) |
T ( other ) T | (3) | |
static_cast< T >( other ) | (4) | |
new T(args, ...) | (5) | |
Class::Class() : member(args, ...) {... | (6) | |
[arg](){... | (7) | (since C++11) |
Explanation
Direct initialization is performed in the following situations:
1) initialization with a nonempty parenthesized list of expressions
2) during list-initialization sequence, if no initializer-list constructors are provided and a matching constructor is accessible, and all necessary implicit conversions are non-narrowing.
3) initialization of a prvalue temporary by functional cast or with a parenthesized expression list
4) initialization of a prvalue temporary by a static_cast expression
5) initialization of an object with dynamic storage duration by a new-expression with a non-empty initializer
6) initialization of a base or a non-static member by constructor initializer list
7) initialization of closure object members from the variables caught by copy in a lambda-expression
The effects of direct initialization are:
- If
Tis a class type, the constructors ofTare examined and the best match is selected by overload resolution. The constructor is then called to initialize the object. - Otherwise, if
Tis a non-class type, standard conversions are used, if necessary, to convert the value of other to the cv-unqualified version ofT.
Notes
Direct-initialization is more permissive than copy-initialization: copy-initialization only considers non-explicit constructors and user-defined conversion functions, while direct-initialization considers all constructors and implicit conversion sequences.
Example
#include <string>
#include <iostream>
#include <memory>
struct Foo {
int mem;
explicit Foo(int n) : mem(n) {}
};
int main()
{
std::string s1("test"); // constructor from const char*
std::string s2(10, 'a');
std::unique_ptr<int> p(new int(1)); // OK: explicit constructors allowed
// std::unique_ptr<int> p = new int(1); // error: constructor is explicit
Foo f(2); // f is direct-initialized:
// constructor parameter n is copy-initialized from the rvalue 2
// f.mem is direct-initialized from the parameter n
// Foo f2 = 2; // error: constructor is explicit
std::cout << s1 << ' ' << s2 << ' ' << *p << ' ' << f.mem << '\n';
}Output:
test aaaaaaaaaa 1 2
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