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# direct initialization

From cppreference.com
< cpp‎ | language

Initializes an object from explicit set of constructor arguments.

### [edit] 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)

### [edit] 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`.

### [edit] 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.

### [edit] 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

### [edit] See also

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

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