Copy assignment operator

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Syntax

Erklrung

Implizit deklarierte Zuweisungsoperator

• Jede direkte Basis B der T hat einen Zuweisungsoperator, deren Parameter B oder const B& oder const volatile B&

  Original:

  each direct base B of T has a copy assignment operator whose parameters are B or const B& or const volatile B&

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• jede nicht-statische Daten Mitglied M der T der Klasse-Typ oder ein Array von Klasse-Typ hat eine Kopie Zuweisungsoperator deren Parameter M oder const M& oder const volatile M&

  Original:

  each non-static data member M of T of class type or array of class type has a copy assignment operator whose parameters are M or const M& or const volatile M&

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Gelschte implizit deklariert Zuweisungsoperator

• T einen nicht-statisches Datenelement, das const ist

  Original:

  T has a non-static data member that is const

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• T hat eine nicht-statische Daten Mitglied einer Referenz-Typ .

  Original:

  T has a non-static data member of a reference type.

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• T hat eine nicht-statische Daten Mitglied, die nicht kopiergeschtzt zugeordnet werden knnen (gelscht hat, unzugnglich oder mehrdeutig Zuweisungsoperator)

  Original:

  T has a non-static data member that cannot be copy-assigned (has deleted, inaccessible, or ambiguous copy assignment operator)

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• T hat direkte oder virtuelle Basisklasse, die nicht kopiergeschtzt zugeordnet werden knnen (gelscht hat, unzugnglich oder mehrdeutig move Zuweisungsoperator)

  Original:

  T has direct or virtual base class that cannot be copy-assigned (has deleted, inaccessible, or ambiguous move assignment operator)

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• T verfgt ber eine benutzerfreundliche erklrt move Konstruktor

  Original:

  T has a user-declared move constructor

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• T verfgt ber eine benutzerfreundliche erklrt move Zuweisungsoperator

  Original:

  T has a user-declared move assignment operator

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Trivial Zuweisungsoperator

• T hat keine virtuelle Member-Funktionen

  Original:

  T has no virtual member functions

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• T hat keine virtuellen Basisklassen

  Original:

  T has no virtual base classes

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• Der Zuweisungsoperator fr jede direkte Basis von T gewhlt ist trivial

  Original:

  The copy assignment operator selected for every direct base of T is trivial

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• Der Zuweisungsoperator fr jeden nicht-statische Klasse-Typ (oder ein Array von Klasse-Typ) memeber der T gewhlt ist trivial

  Original:

  The copy assignment operator selected for every non-static class type (or array of class type) memeber of T is trivial

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Zuweisungsoperator implizit definiert

Notes

Kopieren und tauschen

T& T::operator=(T arg) { // copy/move constructor is called to construct arg swap(arg); // resources exchanged between *this and arg return *this; } // destructor is called to release the resources formerly held by *this

Beispiel

#include <iostream>
#include <memory>
struct A {
    int n;
    std::string s1;
    // user-defined copy assignment, copy-and-swap form
    A& operator=(A other) {
        std::cout << "copy assignment of A\n";
        std::swap(n, other.n);
        std::swap(s1, other.s1);
        return *this;
    }
};

struct B : A {
    std::string s2;
    // implicitly-defined copy assignment
};

struct C {
     std::unique_ptr<int[]> data;
     std::size_t size;
     // non-copy-and-swap assignment
     C& operator=(const C& other) {
         // check for self-assignment
         if(&other == this)
             return *this;
         // reuse storage when possible
         if(size != other.size)
             data.reset(new int[other.size]);
         std::copy(&other.data[0],
                   &other.data[0] + std::min(size, other.size),
                   &data[0]);
         return *this;
     }
     // note: copy-and-swap would always cause a reallocation
};

int main()
{
    A a1, a2;
    std::cout << "a1 = a2 calls ";
    a1 = a2; // user-defined copy assignment

    B b1, b2;
    b2.s1 = "foo";
    b2.s2 = "bar";
    std::cout << "b1 = b2 calls ";
    b1 = b2; // implicitly-defined copy assignment
    std::cout << "b1.s1 = " << b1.s1 << " b1.s2 = " << b1.s2 <<  '\n';
}

a1 = a2 calls copy assignment of A
b1 = b2 calls copy assignment of A
b1.s1 = foo b1.s2 = bar