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# list initialization (since C++11)

From cppreference.com
< cpp‎ | language

Initializes an object from braced-init-list

### [edit] Syntax

#### [edit] direct-list-initialization

T object `{` arg1, arg2, ... `};` | (1) |
T `{` arg1, arg2, ... `};` | (2) |
`new` T `{` arg1, arg2, ... `}` | (3) |
Class `{` T member `{` arg1, arg2, ... `}; };` | (4) |
Class`::`Class`() :` member`{`arg1, arg2, ...`} {...` | (5) |

#### [edit] copy-list-initialization

T object `= {`arg1, arg2, ...`};` | (6) |
function`( {` arg1, arg2, ... `} ) ;` | (7) |
`return {` arg1, arg2, ... `} ;` | (8) |
object`[ {` arg1, arg2, ... `} ] ;` | (9) |
object `=` `{` arg1, arg2, ... `} ;` | (10) |
U`( {` arg1, arg2, ... `} )` | (11) |
Class `{` T member `=` `{` arg1, arg2, ... `}; };` | (12) |

List initialization is performed in the following situations:

- direct-list-initialization (both explicit and non-explicit constructors are considered)

1) initialization of a named variable with a braced-init-list (that is, a possibly empty brace-enclosed list of expressions or nested braced-init-lists)
2) initialization of an unnamed temporary with a braced-init-list
3) initialization of an object with dynamic storage duration with a new-expression, where the initializer is a brace-init-list
4) in a non-static data member initializer that does not use the equals sign
5) in a member initializer list of a constructor if braced-init-list is used

- copy-list-initialization (both explicit and non-explicit constructors are considered, but only non-explicit constructors may be called)

6) initialization of a named variable with a braced-init-list after an equals sign
7) in a function call expression, with braced-init-list used as an argument and list-initialization initializes the function parameter
8) in a return statement with braced-init-list used as the return expression and list-initialization initializes the returned object
9) in a subscript expression with a user-defined `operator[]`, where list-initialization initializes the parameter of the overloaded operator
10) in an assignment expression, where list-initialization initializes the parameter of the overloaded `operator=`
11) functional cast expression or other constructor invocations, where braced-init-list is used in place of a constructor argument. Copy-list-initialization initializes the constructor's parameter (note; the type U in this example is not the type that's being list-initialized; U's constructor's parameter is)
12) in a non-static data member initializer that uses the equals sign

### [edit] Explanation

The effects of list initialization of an object of type `T` are:

If `T` is an aggregate type and the initializer list has a single element of the same or derived type (possibly cv-qualified), the object is initialized from that element (by copy-initialization for copy-list-initialization, or by direct-initialization for direct-list-initialization). Otherwise, if `T` is a character array and the initializer list has a single element that is an appropriately-typed string literal, the array is initialized from the string literal as usual. | (since C++14)

If the braced-init-list is empty and `T` is a class type with a default constructor, value-initialization is performed. Otherwise, if `T` is an aggregate type, aggregate initialization is performed. | (until C++14)
If `T` is an aggregate type, aggregate initialization is performed. Otherwise, If the braced-init-list is empty and `T` is a class type with a default constructor, value-initialization is performed. | (since C++14)

- Otherwise, if `T` is a specialization of std::initializer_list, the `T` object is direct-initialized or copy-initialized, depending on context, from a prvalue of the same type initialized from (until C++17) the braced-init-list.

- Otherwise, the constructors of `T` are considered, in two phases:

  - All constructors that take std::initializer_list as the only argument, or as the first argument if the remaining arguments have default values, are examined, and matched by overload resolution against a single argument of type std::initializer_list

  - If the previous stage does not produce a match, all constructors of `T` participate in overload resolution against the set of arguments that consists of the elements of the braced-init-list, with the restriction that only non-narrowing conversions are allowed. If this stage produces an explicit constructor as the best match for a copy-list-initialization, compilation fails (note, in simple copy-initialization, explicit constructors are not considered at all).

Otherwise, if `T` is a enumeration type that is either scoped or unscoped with fixed underlying type, and if the braced-init-list has only one initializer, and if the conversion from the initializer to the underlying type is non-narrowing, and if the initialization is direct-list-initialization, then the enumeration is initialized with the result of converting the initializer to its underlying type. | (since C++17)

- Otherwise (if `T` is not a class type), if the braced-init-list has only one element andeither `T` isn't a reference type or is a reference type that is compatible with the type of the element, `T` is direct-initialized (in direct-list-initialization) or copy-initialized (in copy-list-initialization), except that narrowing conversions are not allowed.

- Otherwise, if `T` is a reference type that isn't compatible with the type of the element, a temporary of the referenced type is list-initialized, and the reference is bound to that temporary (until C++17)the reference is direct-initialized from a prvalue expression of the referenced type that list-initializes its result object (since C++17). (this fails if the reference is a non-const lvalue reference)

- Otherwise, if the braced-init-list has no elements, `T` is value-initialized.

### [edit] Narrowing conversions

list-initialization limits the allowed implicit conversions by prohibiting the following:

- conversion from a floating-point type to an integer type

- conversion from a long double to double or to float and conversion from double to float, except where the source is a constant expression and overflow does not occur

- conversion from an integer type to a floating-point type, except where the source is a constant expression whose value can be stored exactly in the target type

- conversion from integer or unscoped enumeration type to integer type that cannot represent all values of the original, except where source is a constant expression whose value can be stored exactly in the target type

### [edit] Notes

Every initializer clause is sequenced before any initializer clause that follows it in the braced-init-list. This is in contrast with the arguments of a function call expression, which are unsequenced.

A braced-init-list is not an expression and therefore has no type, e.g. decltype({1,2}) is ill-formed. Having no type implies that template type deduction cannot deduce a type that matches a braced-init-list, so given the declaration template<class T> void f(T); the expression f({1,2,3}) is ill-formed. However, the template parameter can otherwise be deduced, as is the case for std::vector<int> v(std::istream_iterator<int>(std::cin), {}), where the iterator type is deduced by the first argument but also used in the second parameter position. A special exception is made for type deduction using the keyword auto , which deduces any braced-init-list as std::initializer_list.

Also because braced-init-list has no type, special rules for overload resolution apply when it is used as an argument to an overloaded function call.

Aggregates copy/move initialize directly from single-element braced-init-lists of the same type, but non-aggregates consider initializer_list constructors first:
    struct X {
        X() = default;
        X(const X&) = default;
    };

    struct Q {
        Q() = default;
        Q(Q const&) = default;
        Q(std::initializer_list<Q>) {}
    };

    int main() {
      X x;
      X x2 = X { x }; // copy-constructor (not aggregate initialization)
      Q q;
      Q q2 = Q { q }; // initializer-list constructor (not copy constructor)
    }
| (since C++14)

### [edit] Example

Run this code

    #include <iostream>
    #include <vector>
    #include <map>
    #include <string>

    struct Foo {
        std::vector<int> mem = {1,2,3}; // list-initialization of a non-static member
        std::vector<int> mem2;
        Foo() : mem2{-1, -2, -3} {} // list-initialization of a member in constructor
    };

    std::pair<std::string, std::string> f(std::pair<std::string, std::string> p)
    {
        return {p.second, p.first}; // list-initialization in return statement
    }

    int main()
    {
        int n0{};     // value-initialization (to zero)
        int n1{1};    // direct-list-initialization
        std::string s1{'a', 'b', 'c', 'd'}; // initializer-list constructor call
        std::string s2{s1, 2, 2};           // regular constructor call
        std::string s3{0x61, 'a'}; // initializer-list ctor is preferred to (int, char)

        int n2 = {1}; // copy-list-initialization
        double d = double{1.2}; // list-initialization of a temporary, then copy-init

        std::map<int, std::string> m = { // nested list-initialization
               {1, "a"},
               {2, {'a', 'b', 'c'} },
               {3, s1}
        };

        std::cout << f({"hello", "world"}).first // list-initialization in function call
                  << '\n';

        const int (&ar)[2] = {1,2}; // binds a lvalue reference to a temporary array
        int&& r1 = {1}; // binds a rvalue reference to a temporary int
    //  int& r2 = {2}; // error: cannot bind rvalue to a non-const lvalue ref

    //  int bad{1.0}; // error: narrowing conversion
        unsigned char uc1{10}; // okay
    //  unsigned char uc2{-1}; // error: narrowing conversion

        Foo f;

        std::cout << n0 << ' ' << n1 << ' ' << n2 << '\n'
                  << s1 << ' ' << s2 << ' ' << s3 << '\n';
        for(auto p: m)
            std::cout << p.first << ' ' << p.second << '\n';
        for(auto n: f.mem)
            std::cout << n << ' ';
        for(auto n: f.mem2)
            std::cout << n << ' ';
    }

Output:

    world
    0 1 1
    abcd cd aa
    1 a
    2 abc
    3 abcd
    1 2 3 -1 -2 -3

### [edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
CWG 1467 | C++14 | same-type initialization of aggregates and char arrays was prohibited | same-type initialization allowed
CWG 1467 | C++14 | std::initializer_list constructors had priority over copy constructors for single-element lists | single-element lists initialize directly

### [edit] See also

- default initialization
- direct initialization
- aggregate initialization
- copy initialization

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