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direct initialization

Initializes an object from explicit set of constructor arguments.

Syntax

T object ( arg );

T object ( arg1, arg2, ... );

 (1)
T object { arg };

T object { arg1, arg2, ... };

 (2) (since C++11)
T ( other )

T ( arg1, arg2, ... );

 (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 T is a class type,
  • if the initializer is a prvalue expression whose cv-unqualified type is the same class as T, the initializer expression itself, rather that a temporary materialized from it, is used to initialize the destination object: see copy elision
(since C++17)
  • the constructors of T are examined and the best match is selected by overload resolution. The constructor is then called to initialize the object.
  • Otherwise, if T is a non-class type, standard conversions are used, if necessary, to convert the value of other to the cv-unqualified version of T.

Notes

Direct-initialization is more permissive than copy-initialization: copy-initialization only considers non-explicit constructors and non-explicit user-defined conversion functions, while direct-initialization considers all constructors and all user-defined conversion functions.

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

See also

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