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const_cast conversion

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const_cast conversion
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Konvertiert zwischen Typen mit unterschiedlichen cv-Qualifikation .
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Converts between types with different cv-qualification.
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Inhaltsverzeichnis

[Bearbeiten] Syntax

const_cast < new_type > ( expression )
Gibt einen Wert vom Typ new_type .
Original:
Returns a value of type new_type.
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[Bearbeiten] Erklärung

Nur die folgenden Konvertierungen können mit const_cast durchgeführt werden. Insbesondere darf nur const_cast verwendet zu werfen (entfernen) Konstantheit oder Volatilität werden .
Original:
Only the following conversions can be done with const_cast. In particular, only const_cast may be used to cast away (remove) constness or volatility.
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1)
Zwei möglicherweise mehrstufige Zeiger auf die gleiche Art können untereinander konvertiert werden, unabhängig von cv-Qualifikation auf jeder Ebene .
Original:
Two possibly multilevel pointers to the same type may be converted between each other, regardless of cv-qualifiers at each level.
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2)
Lvalue aller Art T kann zu einem L-Wert oder rvalue Verweis auf die gleiche Art T, mehr oder weniger qualifizierte cv umgewandelt werden. Ebenso kann eine rvalue zu einer mehr oder weniger cv qualifizierte rvalue Verweis umgewandelt werden .
Original:
lvalue of any type T may be converted to a lvalue or rvalue reference to the same type T, more or less cv-qualified. Likewise, an rvalue may be converted to a more or less cv-qualified rvalue reference.
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3)
Es gelten dieselben Regeln, um möglicherweise mehrstufige Zeiger auf Daten Mitgliedern .
Original:
Same rules apply to possibly multilevel pointers to data members.
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4)
Null-Zeiger-Wert kann auf den Null-Zeiger-Wert von new_type umgewandelt werden
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null pointer value may be converted to the null pointer value of new_type
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Wie bei allen Besetzung Ausdrücke, ist das Ergebnis:
Original:
As with all cast expressions, the result is:
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  • ein L-Wert, wenn new_type ist ein L-Wert Referenz-Typ oder ein rvalue Bezug auf Art funktionieren;
    Original:
    an lvalue if new_type is an lvalue reference type or an rvalue reference to function type;
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  • ein xWert wenn new_type ist ein rvalue Verweis auf Objekttyp;
    Original:
    an xvalue if new_type is an rvalue reference to object type;
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  • a prvalue sonst .
    Original:
    a prvalue otherwise.
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[Bearbeiten] Notes

Zeiger auf Funktionen und Zeiger auf Member-Funktionen unterliegen nicht const_cast
Original:
Pointers to functions and pointers to member functions are not subject to const_cast
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Obwohl const_cast kann Konstantheit von jedem Zeiger oder eine Referenz zu entfernen, mit der daraus resultierenden Zeiger oder eine Referenz auf ein Objekt, das erklärt const aufruft undefined Verhalten wurde schreiben .
Original:
Even though const_cast may remove constness from any pointer or reference, using the resulting pointer or reference to write to an object that was declared const invokes undefined behavior.
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[Bearbeiten] Keywords

const_cast

[Bearbeiten] Beispiel

#include <iostream>
 
struct type {
    type() :i(3) {}
    void m1(int v) const {
        // this->i = v;                 // compile error: this is a pointer to const
        const_cast<type*>(this)->i = v; // OK
    }
    int i;
};
 
int main() 
{
    int i = 3;                    // i is not declared const
    const int& cref_i = i; 
    const_cast<int&>(cref_i) = 4; // OK: modifies i
    std::cout << "i = " << i << '\n';
 
    type t;
    t.m1(4);
    std::cout << "type::i = " << t.i << '\n';
 
    const int j = 3; // j is declared const
    int* pj = const_cast<int*>(&j);
    *pj = 4;         // undefined behavior!
 
    void (type::*mfp)(int) const = &type::m1; // pointer to member function
//  const_cast<void(type::*)(int)>(mfp); // compiler error: const_cast does not
                                         // work on function pointers
}

Output:

i = 4
type::i = 4

[Bearbeiten] Siehe auch