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    <p>Your implementation is not good : the <code>operator=</code> should return reference to object of type Array, and should not be virtual :</p> <pre><code>Array &amp;Array::operator=(const DataStructure &amp;other) { DataStructure::operator=(other); return *this; } </code></pre> <p>You can change <code>DataStructure</code> to use <a href="http://en.wikipedia.org/wiki/Non-virtual_interface_pattern" rel="nofollow">NVI</a> :</p> <pre><code>#include &lt;iostream&gt; class DataStructure { public: DataStructure(){} inline virtual ~DataStructure(){} DataStructure &amp;operator=(const DataStructure &amp; other); inline friend const DataStructure &amp;operator+(const int a, const DataStructure &amp; other) { other.add(a); return other; } //... private: virtual void add( const int a ) const = 0; }; struct Array : DataStructure { virtual void add( const int a ) const { std::cout&lt;&lt;"adding "&lt;&lt;a&lt;&lt;std::endl; } }; void foo(const DataStructure &amp;a) { const DataStructure &amp;b = 5 + a; } int main() { Array a; foo(a); } </code></pre> <p>Check <a href="http://codepad.org/LaCMeP65" rel="nofollow">live demo</a>. Then you have to implement the method <code>add</code> in your derived class Array.</p> <hr> <p>To your edit :</p> <p>Your new idea is an awful way of doing things in c++. What you do there is tell your compiler : "stop complaining, I know what I am doing". Also, it causing an undefined behaviour. It may appear to work, until you application starts crashing one day.</p>
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    1. POc++: Inheriting a method with a different (but covariant) return type
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    1. CO@stringQNA Your question looks like [xy problem](http://meta.stackexchange.com/questions/66377/what-is-the-xy-problem)
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    2. COI don't see how it makes the code any shorter - I still have to implement 2 methods for any method that has `DataStrucure` in its return type. Also, in my original code (not presented here) `Array` isn't the only class inheriting from `DataStrucure`, so it makes more sense to implement the method's main operation in the base class `DataStrucure` and just convert the result in the derived classes.
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