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POHow does the abstract base class avoid the partial assignment?
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copied!<p>As is talked about in Item 33 in "More Effective C++", the assignment problem is</p> <pre><code>//Animal is a concrete class Lizard:public Animal{}; Chicken:public Animal{}; Animal* pa=new Lizard('a'); Animal* pb=new Lizard('b'); *pa=*pb;//partial assignment </code></pre> <p>However, if I define <code>Animal</code> as an abstract base class, we can also compile and run the sentence:<code>*pa=*pb</code>. Partial assignment problem is still there.<br> See my example:</p> <pre><code>#include <iostream> class Ab{ private: int a; double b; public: virtual ~Ab()=0; }; Ab::~Ab(){} class C:public Ab{ private: int a; double b; }; class D:public Ab{ private: int a; double b; }; int main() { Ab *pc=new C(); Ab *pd=new D(); *pc=*pd; return 0; } </code></pre> <p>Do I miss something? Then what's the real meaning of the abstract base class?</p> <p><strong>I got the answer by myself. I missed a code snippet in the book.</strong><br> Use protected <code>operator=</code> in the base class to avoid <code>*pa=*pb</code>. Use abstract base class to avoid <code>animal1=animal2</code>.Then the only allowed expressions are <code>lizard1=lizard2;chicken1=chicken2;</code><br> See the code below: </p> <pre><code>#include <iostream> class Ab{ private: int a; double b; public: virtual ~Ab()=0; protected: //!!!!This is the point Ab& operator=(const Ab&){...} }; Ab::~Ab(){} class C:public Ab{ public: C& operator=(const C&){...} private: int a; double b; }; class D:public Ab{ public: D& operator=(const D&){...} private: int a; double b; }; int main() { Ab *pc=new C(); Ab *pd=new D(); *pc=*pd; return 0; } </code></pre>

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