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# operator= for derived class

 P: n/a struct A{ A& operator=( const A& a ){ i = a.i; return *this; } private: int i; }; struct B : public A{ B& operator=( const B& b ){ j = b.j; return *this; } // how to copy i? private: int j; }; In the above code, what would be the standard way of coping A::i in B::operator= ? Jul 22 '05 #1
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 P: n/a MojoRison wrote: struct A{ A& operator=( const A& a ){ i = a.i; return *this; } private: int i; }; struct B : public A{ B& operator=( const B& b ){ j = b.j; return *this; } // how to copy i? private: int j; }; In the above code, what would be the standard way of coping A::i in B::operator= ? For the classes in their presented form, there is no need for operator=. But you probably already knew that. The simple way is to call the A::operator= directly inside B::operator= : B& operator=(const B& b) { A::operator=(b); j = b.j; return *this } Victor Jul 22 '05 #2

 P: n/a MojoRison wrote: struct A{ A& operator=( const A& a ){ i = a.i; return *this; } private: int i; }; struct B : public A{ B& operator=( const B& b ){ j = b.j; return *this; } // how to copy i? private: int j; }; In the above code, what would be the standard way of coping A::i in B::operator= ? Firstly, for such classes you don't need to implement the copy assignment operator explicitly. The one automatically provided by the compiler will do exactly what you want. If you still want to implement these operators manually, then the proper way to do it would be calling the inherited copy assignment operator ('A::operator=') from B's implementation B& operator=( const B& b ) { A::operator=(b); j = b.j; return *this; } -- Best regards, Andrey Tarasevich Jul 22 '05 #3

 P: n/a MojoRison wrote: struct A{ A& operator=( const A& a ){ i = a.i; return *this; } private: int i; }; struct B : public A{ B& operator=( const B& b ){ j = b.j; return *this; } // how to copy i? private: int j; }; In the above code, what would be the standard way of coping A::i in B::operator= ? I would make the operator= in the parent class protected. This would prevent "slicing", where only the parent portion is copied. See  below. One method of implementing assignment operator in child classes is: Child& Child :: operator=(const Child& ch) { if (this != &child) /* guard against self assignment */ { (Parent&)(*this) = (Parent&)(ch); /* assignment of child members */ } return *this; } The above fragment comes from Effective C++ or More Effective C++ by Scott Meyers.  When parents allow public assignment, one can assign the parent portion of child A to child B, using pointers to the parent class. struct Parent { Parent& operator=(const Parent& p); }; struct Son : Parent { Son& operator=(const Son& s); }; struct Daughter : Parent { Daughter& operator=(const Daughter& d); }; /* ... */ Son bill; Son jack; Daughter amy; Parent * p_male = &bill; Parent * p_female = &amy; Parent * p_brother = & jack; *p_brother = *p_male; /* Are the child members copied? */ /* One _might_ think so... */ *p_male = *p_female; /* copies only parent portion */ /* Do you want this allowed? */ This has cost me many hours of debugging, so I've learned to make assignment operators of base classes as protected. to prevent the above scenario from occurring. -- Thomas Matthews C++ newsgroup welcome message: http://www.slack.net/~shiva/welcome.txt C++ Faq: http://www.parashift.com/c++-faq-lite C Faq: http://www.eskimo.com/~scs/c-faq/top.html alt.comp.lang.learn.c-c++ faq: http://www.comeaucomputing.com/learn/faq/ Other sites: http://www.josuttis.com -- C++ STL Library book http://www.sgi.com/tech/stl -- Standard Template Library Jul 22 '05 #4

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