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copy initialization

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Initialisierung
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Initialization
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Initialisiert ein Objekt von einem anderen Objekt
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Initializes an object from another object
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Inhaltsverzeichnis

[Bearbeiten] Syntax

T object = other ; (1)
f(other); (2)
return other; (3)
catch ( T other) ; (4)
T array [ N ] = { other }; (5)

[Bearbeiten] Erklärung

Copy-Initialisierung wird in den folgenden Situationen ausgeführt:
Original:
Copy initialization is performed in the following situations:
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1)
wenn eine benannte Variable (automatische, statische oder threadlokale) mit der Initialisierung aus erklärt wird ein Gleichheitszeichen gefolgt von einem Ausdruck .
Original:
when a named variable (automatic, static, or thread-local) is declared with the initializer consisting of an equals sign followed by an expression.
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2)
Wenn ein Argument an eine Funktion Wert
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when passing an argument to a function by value
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3)
bei der Rückkehr aus einer Funktion, die durch einen Wert zurückgibt
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when returning from a function that returns by value
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4)
wenn das Fangen einer Ausnahme nach Wert
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when catching an exception by value
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5)
als Teil aggregierten Initialisierung, zu initialisieren jedes Element, für das eine Initialisierung bereitgestellt wird
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as part of aggregierten Initialisierung, to initialize each element for which an initializer is provided
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Die Auswirkungen der Kopie der Initialisierung sind:
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The effects of copy initialization are:
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  • Wenn T ist eine Klasse-Typ und der Art der other wird cv-unqualifizierte Version des T oder einer Klasse von T abgeleitet, die Konstrukteure von T werden untersucht und die beste Übereinstimmung wird durch Überlast Auflösung gewählt. Der Konstruktor wird dann aufgerufen, um das Objekt zu initialisieren .
    Original:
    If T is a class type and the type of other is cv-unqualified version of T or a class derived from T, the constructors of T are examined and the best match is selected by overload resolution. The constructor is then called to initialize the object.
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  • Wenn T ist eine Klasse-Typ, und die Art der other verschieden ist, oder wenn T Nicht-Klasse-Typ ist, sondern der Typ der other ist ein Klasse-Typ, Umwandlung Sequenzen vom Benutzer definiert, die aus der Art der zu other T umwandeln kann untersucht werden und die beste ein durch Überlastung Auflösung gewählt. Das Ergebnis der Umwandlung, die eine temporäre prvalue der Zieltyp ist, wird dann verwendet, um das Objekt leiten zu initialisieren. Der letzte Schritt besteht in der Regel eliminiert und das Ergebnis der Konvertierung wird direkt in dem Speicher für das Zielobjekt zugeordnet konstruiert, aber die Kopie Konstruktors muss zugänglich sein, obwohl dies nicht der Fall verwendet .
    Original:
    If T is a class type, and the type of other is different, or if T is non-class type, but the type of other is a class type, Umwandlung Sequenzen vom Benutzer definiert that can convert from the type of other to T are examined and the best one is selected through overload resolution. The result of the conversion, which is a prvalue temporary of the destination type, is then used to leiten zu initialisieren the object. The last step is usually eliminiert and the result of the conversion function is constructed directly in the memory allocated for the target object, but the copy constructor is required to be accessible even though it's not used.
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  • Andernfalls (wenn weder T noch die Art des other sind Klassenarten) werden Standard-Konvertierungen verwendet, falls notwendig, den Wert der an die other cv-uneingeschränkten Version T umzuwandeln .
    Original:
    Otherwise (if neither T nor the type of other are class types), Standard-Konvertierungen are used, if necessary, to convert the value of other to the cv-unqualified version of T.
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[Bearbeiten] Notes

Copy-Initialisierung ist weniger permissive als direkte Initialisierung: copy-Initialisierung nur hält nicht explizit Konstruktoren und benutzerdefinierte Konvertierungsfunktionen .
Original:
Copy-initialization is less permissive than direct-initialization: copy-initialization only considers non-explicit constructors and user-defined conversion functions.
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Wenn other ein rvalue Ausdruck ist, wird bewegen Konstruktor durch Überlastung Auflösung ausgewählt und aufgerufen werden während des Kopierens-Initialisierung .
Original:
If other is an rvalue expression, bewegen Konstruktor will be selected by overload resolution and called during copy-initialization.
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Das Gleichheitszeichen, =, in copy-Initialisierung einer benannten Variable ist nicht mit dem Zuweisungsoperator verwandt. Zuweisungsoperator Überlastungen haben keinen Einfluss auf copy-Initialisierung .
Original:
The equals sign, =, in copy-initialization of a named variable is not related to the assignment operator. Assignment operator overloads have no effect on copy-initialization.
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[Bearbeiten] Beispiel

#include <string>
#include <utility>
#include <memory>
 
int main()
{
    std::string s = "test"; // OK: constructor is non-explicit
    std::string s2 = std::move(s); // this copy-initialization performs a move
 
//  std::unique_ptr<int> p = new int(1); // error: constructor is explicit
    std::unique_ptr<int> p(new int(1)); // OK: direct-initialization
 
    int n = 3.14;    // floating-integral conversion
    const int b = n; // const doesn't matter
    int c = b;       // ...either way
}


[Bearbeiten] Siehe auch