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list initialization

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Miscellaneous
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Initialisiert ein Objekt aus braced-init-Liste
Original:
Initializes an object from braced-init-list
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Inhaltsverzeichnis

[Bearbeiten] Syntax

T object { arg1, arg2, ... }; (1)
T { arg1, arg2, ... }; (2)
new T { arg1, arg2, ... }; (3)
return { arg1, arg2, ... } ; (4)
function( { arg1, arg2, ... } ) ; (5)
object[ { arg1, arg2, ... } ] ; (6)
T( { arg1, arg2, ... } ) (7)
Class { T member = { arg1, arg2, ... }; }; (8)
Class::Class() : member{arg1, arg2, ...} {... (9)
T object = {arg1, arg2, ...}; (10)

[Bearbeiten] Erklärung

Liste der Initialisierung wird in den folgenden Situationen ausgeführt:
Original:
List initialization is performed in the following situations:
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1)
Initialisierung einer benannten Variable mit einem Doppelpack eingeschlossenen Liste von Ausdrücken oder verschachtelte Listen (verspannt-init-Liste)
Original:
initialization of a named variable with a brace-enclosed list of expressions or nested lists (braced-init-list)
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2)
Initialisierung eines ungenannten temporären mit einer verspannt-init-Liste
Original:
initialization of an unnamed temporary with a braced-init-list
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3)
Initialisierung eines Objekts mit dynamischen Lagerdauer mit einem neuen Ausdruck, wo die Initialisierung ist eine Klammer-init-Liste
Original:
initialization of an object with dynamic storage duration with a new-expression, where the initializer is a brace-init-list
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4)
in einem return Anweisung mit braced-init-Liste als return-Ausdruck verwendet
Original:
in a return statement with braced-init-list used as the return expression
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5)
in einem Aufruf der Funktion Ausdruck, mit verspannten-init-list als Argument verwendet
Original:
in a function call expression, with braced-init-list used as an argument
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6)
in einem Index Ausdruck mit einem benutzerdefinierten Operator []
Original:
in a subscript expression with a user-defined operator[]
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7)
in einem funktionalen Cast-Ausdruck oder andere direkte Initialisierung, mit verspannten-init-Liste als Konstruktorargument verwendet
Original:
in a functional cast expression or other direct-initialization, with braced-init-list used as the constructor argument
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8)
in einer nicht-statische Daten Mitglied Initialisierung
Original:
in a non-static data member initializer
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9)
in einem Konstruktor Initialisierungsliste
Original:
in a constructor initializer list
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10)
auf der rechten-Hand-Seite des Gleichheitszeichens (ähnlich copy-Initialisierung)
Original:
on the right-hand-side of the equals sign (similar to copy-Initialisierung)
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Die Wirkungen der Liste Initialisierung eines Objekts vom Typ T sind:
Original:
The effects of list initialization of an object of type T are:
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  • Wenn die braced-init-Liste ist leer und T ist eine Klasse-Typ mit einem Standardkonstruktor wird Value-Initialisierung durchgeführt .
    Original:
    If the braced-init-list is empty and T is a class type with a default constructor, Value-Initialisierung is performed.
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  • Andernfalls, wenn T ist eine Spezialisierung der std::initializer_list eine neue std::initializer_list Objekt des gleichen Typs gebaut und eingesetzt zu lenken initialisieren oder kopieren initialisieren das Objekt vom Typ T, abhängig vom Kontext .
    Original:
    Otherwise, if T is a specialization of std::initializer_list, a new std::initializer_list object of the same type is constructed and used to direct-initialize or copy-initialize the object of type T, depending on context.
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  • Andernfalls werden die constuctors der T betrachtet, in zwei Phasen:
    Original:
    Otherwise, the constuctors of T are considered, in two phases:
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  • Alle Konstruktoren, std::initializer_list nehmen als einziges Argument oder als erstes Argument, wenn die restlichen Argumente Standardwerte haben, werden geprüft und abgestimmt durch Überlastung Resolution gegen ein einzelnes Argument vom Typ std::initializer_list
    Original:
    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
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  • Wenn der vorherigen Stufe nicht produziert ein Spiel, alle Konstruktoren T in Überlast Beschlussfassung teilnehmen gegen die Reihe von Argumenten, die aus den Elementen der braced-init-Liste besteht, mit der Einschränkung, dass nur einschränkende Konvertierungen erlaubt sind. Wenn dieses Stadium produziert einen expliziten Konstruktor als beste Übereinstimmung für eine Kopie-list-Initialisierung, Erstellung fehlschlägt (beachten Sie, in einfachen Copy-Initialisierung werden explizite Konstruktoren überhaupt nicht berücksichtigt)
    Original:
    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 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)
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  • Andernfalls, wenn T Referenz-Typ ist, ist ein prvalue vorübergehende des referenzierten Typs list-initialisiert, und der Verweis auf diese temporären gebunden .
    Original:
    Otherwise, if T is reference type, a prvalue temporary of the referenced type is list-initialized, and the reference is bound to that temporary.
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  • Andernfalls, wenn die verspannt-init-Liste hat nur ein Element, T ist direkte initialisiert oder copy-initialisiert, abhängig vom Kontext, außer dass einschränkende Konvertierungen sind nicht erlaubt .
    Original:
    Otherwise, if the braced-init-list has only one element, T is direkte initialisiert or copy-initialisiert, depending on context, except that narrowing conversions are not allowed.
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  • Andernfalls, wenn die verspannt-init-Liste keine Elemente, T ist Wert initialisiert .
    Original:
    Otherwise, if the braced-init-list has no elements, T is Wert initialisiert.
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[Bearbeiten] Einschränkende Konvertierungen

list-Initialisierung begrenzt die zulässige impliziten Konvertierungen durch das Verbot der folgenden:
Original:
list-initialization limits the allowed impliziten Konvertierungen by prohibiting the following:
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  • Umwandlung von einer Floating-Point-Typ in einen Integer-Typ
    Original:
    conversion from a floating-point type to an integer type
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  • Umwandlung von einer long double um double oder float und Umwandlung von double um float, sofern nicht die Quelle ist ein konstanter Ausdruck dessen Wert genau in den Ziel-Typ gespeichert werden
    Original:
    conversion from a long double to double or to float and conversion from double to float, except where the source is a constant expression whose value can be stored exactly in the target type
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  • Umwandlung von ganzzahligen oder ohne Bereichseinschränkung Enumerationstyp um Ganzzahl, die sich nicht darstellen können alle Werte des ursprünglichen, sofern Quelle ist ein konstanter Ausdruck dessen Wert genau in den Ziel-Typ gespeichert werden
    Original:
    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
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[Bearbeiten] Notes

Verspannt-init-Liste ist nicht ein Ausdruck und hat keinen Typ für sich: zum Beispiel beim Aufruf einer Funktion Vorlage verspannt-init-list Argument kann nicht für Vorlagentyp Abzug verwendet werden. Eine besondere Ausnahme für das Schlüsselwort auto, die jede verspannt-init-Liste als std::initializer_list leitet gemacht .
Original:
Braced-init-list is not an expression and has no type on its own: for example, when calling a function template, braced-init-list argument cannot be used for template type deduction. A special exception is made for the keyword auto, which deduces any braced-init-list as std::initializer_list.
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[Bearbeiten] Beispiel

#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


[Bearbeiten] Siehe auch