# direct initialization

From cppreference.com
< cpp‎ | language

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)
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 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 a[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: 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}, {});
    }

Similarly, in the case of an ambiguity between a expression statement with a function-style cast expression (3) as its leftmost subexpression and a declaration statement, the ambiguity is resolved by treating it as a declaration. This disambiguation is purely syntactic: it doesn't consider the meaning of names occurring in the statement other than whether they are type names.

    struct M { };
    struct L { L(M&); };

    M n;
    void f() {
        M(m); // declaration, equivalent to M m;
        L(n); // ill-formed declaration
        L(l)(m); // still a declaration
    }

### 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

- default initialization
- copy initialization
- aggregate initialization
- list initialization

Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/language/direct_initialization&oldid=109572"