Direct initialization

 

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Initialization

Initializer

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Value initialization

Direct initialization

Copy initialization

List initialization (C++11)

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Initializes an object from explicit set of constructor arguments.

Syntax

T object ( arg ); T object ( arg1, arg2, ... );	(1)
T object { arg };	(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 or braced-init-lists (since C++11)

2) initialization of an object of non-class type with a single brace-enclosed initializer (note: for class types and other uses of braced-init-list, see list-initialization) (since C++11)

3) initialization of a prvalue temporary (until C++17)the result object of a prvalue (since C++17) by function-style cast or with a parenthesized expression list

4) initialization of a prvalue temporary (until C++17)the result object of a prvalue (since C++17) 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 an array type,

the program is ill-formed.	(until C++20)
the array is initialized as in aggregate initialization, except that narrowing conversions are allowed and any elements without an initializer are value-initialized. struct A { explicit A(int i = 0) {} };   A a[2](A(1)); // OK: initializes a[0] with A(1) and a[1] with A() A b[2]{A(1)}; // error: implicit copy-list-initialization of b[1] // from {} selected explicit constructor	(since C++20)

• If T is a class type,

if the initializer is a prvalue expression whose type is the same class as T (ignoring cv-qualification), the initializer expression itself, rather than a temporary materialized from it, is used to initialize the destination object. (Before C++17, the compiler may elide the construction from the prvalue temporary in this case, but the appropriate constructor must still be accessible: 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 the destination type is a (possibly cv-qualified) aggregate class, it is initialized as described in aggregate initialization except that narrowing conversions are permitted, designated initializers are not allowed, a temporary bound to a reference does not have its lifetime extended, there is no brace elision, and any elements without an initializer are value-initialized. struct B { int a; int&& r; };   int f(); int n = 10;   B b1{1, f()}; // OK, lifetime is extended B b2(1, f()); // well-formed, but dangling reference B b3{1.0, 1}; // error: narrowing conversion B b4(1.0, 1); // well-formed, but dangling reference B b5(1.0, std::move(n)); // OK

(since C++20)

• Otherwise, if T is a non-class type but the source type is a class type, the conversion functions of the source type and its base classes, if any, are examined and the best match is selected by overload resolution. The selected user-defined conversion is then used to convert the initializer expression into the object being initialized.
• Otherwise, if T is bool and the source type is std::nullptr_t, the value of the initialized object is false.
• Otherwise, standard conversions are used, if necessary, to convert the value of other to the cv-unqualified version of T, and the initial value of the object being initialized is the (possibly converted) value.

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.

In case of ambiguity between a variable declaration using the direct-initialization syntax (1) (with round parentheses) and a function declaration, the compiler always chooses function declaration. This disambiguation rule is sometimes counter-intuitive and has been called the most vexing parse.

#include <iterator>
#include <string>
#include <fstream>
int main()
{
    std::ifstream file("data.txt");
    // the following is a function declaration:
    std::string str(std::istreambuf_iterator<char>(file),
                    std::istreambuf_iterator<char>());
    // it declares a function called str, whose return type is std::string,
    // first parameter has type std::istreambuf_iterator<char> and the name "file"
    // second parameter has no name and has type std::istreambuf_iterator<char>(),
    // which is rewritten to function pointer type std::istreambuf_iterator<char>(*)()
 
    // pre-C++11 fix: extra parentheses around one of the arguments
    std::string str( (std::istreambuf_iterator<char>(file) ),
                      std::istreambuf_iterator<char>());
 
    // post-C++11 fix: list-initialization for any of the arguments
    std::string str(std::istreambuf_iterator<char>{file}, {});
}

Example

Run this code

#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

• copy elision
• converting constructor
• copy assignment
• copy constructor
• default constructor
• destructor
• explicit
• initialization
  • aggregate initialization
  • constant initialization
  • copy initialization
  • default initialization
  • initializer list
  • list initialization
  • reference initialization
  • value initialization
  • zero initialization
• move assignment
• move constructor
• new