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    <p>I think the exact terminology for what you need is "template covariance", meaning that if B inherits from A, then somehow <code>T&lt;B&gt;</code> inherits from <code>T&lt;A&gt;</code>. This is not the case in C++, nor it is with Java and C# generics*.</p> <p>There is a good reason to avoid template covariance: this will simply remove all type safety in the template class. Let me explain with the following example:</p> <pre><code>//Assume the following class hierarchy class Fruit {...}; class Apple : public Fruit {...}; class Orange : public Fruit {...}; //Now I will use these types to instantiate a class template, namely std::vector int main() { std::vector&lt;Apple&gt; apple_vec; apple_vec.push_back(Apple()); //no problem here //If templates were covariant, the following would be legal std::vector&lt;Fruit&gt; &amp; fruit_vec = apple_vec; //push_back would expect a Fruit, so I could pass it an Orange fruit_vec.push_back(Orange()); //Oh no! I just added an orange in my apple basket! } </code></pre> <p>Consequently, you should consider <code>T&lt;A&gt;</code> and <code>T&lt;B&gt;</code> as completely unrelated types, regardless of the relation between A and B.</p> <p>So how could you solve the issue you're facing? In Java and C#, you could use respectively <em>bounded wildcards</em> and <em>constraints</em>:</p> <pre><code>//Java code Bar(Container&lt;? extends Interface) {...} //C# code Bar&lt;T&gt;(Container&lt;T&gt; container) where T : Interface {...} </code></pre> <p>The next C++ Standard (known as C++1x (formerly C++0x)) initially contained an even more powerful mechanism named <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2005/n1758.pdf" rel="nofollow noreferrer">Concepts</a>, that would have let developers enforce syntaxic and/or semantic requirements on template parameters, but was unfortunately postponed to a later date. However, Boost has a <a href="http://www.boost.org/doc/libs/1_42_0/libs/concept_check/concept_check.htm" rel="nofollow noreferrer">Concept Check library</a> that may interest you.</p> <p>Nevertheless, concepts might be a little overkill for the problem you encounter, an using a simple static assert as proposed by <a href="https://stackoverflow.com/questions/2203388/c-templates-polymorphism/2203439#2203439">@gf</a> is probably the best solution.</p> <p><sub>* Update: Since .Net Framework 4, it is possible to mark generic parameters has being <a href="http://msdn.microsoft.com/en-us/library/dd799517.aspx" rel="nofollow noreferrer">covariant or contravariant</a>.</sub></p>
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