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default initialization

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Bietet die Standard-Ausgangswert zu einem neuen Objekt .
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Provides the default initial value to a new object.
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Inhaltsverzeichnis

[Bearbeiten] Syntax

T object ; (1)
new T ; (2)

[Bearbeiten] Erklärung

Default-Initialisierung ist in drei Situationen durchgeführt:
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Default initialization is performed in three situations:
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1)
wenn eine Variable mit automatischer Lagerdauer wird ohne Initialisierung deklariert
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when a variable with automatic storage duration is declared with no initializer
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2)
wenn ein Objekt mit dynamischen Lagerdauer wird durch einen neuen Ausdruck ohne Initialisierung erstellt
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when an object with dynamic storage duration is created by a new-expression without an initializer
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3)
wenn eine Basisklasse oder ein nicht-statisches Datenelement nicht in einem Konstruktor Initialisierungsliste erwähnt und dass Konstruktor aufgerufen wird .
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when a base class or a non-static data member is not mentioned in a constructor Initialisierungsliste and that constructor is called.
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Die Auswirkungen der Default-Initialisierung sind:
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The effects of default initialization are:
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  • Wenn T eine Klasse-Typ ist, wird die Standardkonstruktor aufgerufen, um den ursprünglichen Wert für das neue Objekt bietet .
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    If T is a class type, the Standardkonstruktor is called to provide the initial value for the new object.
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  • Wenn T ein Array-Typ ist, ist jedes Element des Arrays standardmäßig initialisiert .
    Original:
    If T is an array type, every element of the array is default-initialized.
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  • Ansonsten wird nichts getan .
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    Otherwise, nothing is done.
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Wenn T a const qualifizierten Typ ist, muss es eine Klasse-Typ mit einem vom Benutzer bereitgestellten Standardkonstruktor sein .
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If T is a const-qualified type, it must be a class type with a user-provided default constructor.
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[Bearbeiten] Notes

Standard Initialisierung nicht Klassenvariablen mit automatischer und dynamischer Lagerdauer produziert Objekten mit unbestimmter (statische und Faden-locale Objekte erhalten Null initialisiert)
Original:
Default initialization of non-class variables with automatic and dynamic storage duration produces objects with indeterminate values (static and thread-locale objects get Null initialisiert)
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Referenz nicht default-initialisiert werden .
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Reference cannot be default-initialized.
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[Bearbeiten] Beispiel

#include <string>
struct T1 {};
class T2 { 
    int mem;
 public:
    T2() {} // "mem" not in initializer list
};
int n; // This is not default-initialization, the value is zero.
int main()
{
    int n;            // non-class: the value is undeterminate
    std::string s;    // calls default ctor, the value is "" (empty string)
    std::string a[2]; // calls default ctor, creates two empty strings
//    int& r;         // error: default-initializing a reference
//    const int n;    // error: const non-class type
//    const T1 nd;    // error: const class type with implicit ctor
    T1 t1; // ok, calls implicit default ctor
    const T2 t2; // ok, calls the user-provided default ctor 
                 // t2.mem is default-initialized
}


[Bearbeiten] Siehe auch