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POResolving a Circular Dependency between Template Classes
 

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    <p>I have two classes, <code>Foo&lt;T&gt;</code> and <code>Bar&lt;T&gt;</code>, derived from <code>Base</code>. Each overrides a method <code>virtual Base* convert(ID) const</code>, where <code>ID</code> is an instance of a type that uniquely identifies a particular instantiation of <code>Foo</code> or <code>Bar</code> (pretend it's an <code>enum</code>). The problem is that <code>Foo::convert()</code> needs to be able to return a <code>Bar</code> instance, and likewise <code>Bar::convert()</code> needs to be able to instantiate <code>Foo</code>. Since they're both templates, this results in a circular dependency between <code>Foo.h</code> and <code>Bar.h</code>. How do I resolve this?</p> <p><strong>Edit:</strong> A forward declaration does not work because the implementation of each method needs the constructor of the other class:</p> <p><code>Foo.h</code>:</p> <pre><code>#include &lt;Base.h&gt; template&lt;class T&gt; class Bar; template&lt;class T&gt; class Foo : public Base { ... }; template&lt;class T&gt; Base* Foo&lt;T&gt;::convert(ID id) const { if (id == BAR_INT) return new Bar&lt;int&gt;(value); // Error. ... } </code></pre> <p><code>Bar.h</code>:</p> <pre><code>#include &lt;Base.h&gt; template&lt;class T&gt; class Foo; template&lt;class T&gt; class Bar : public Base { ... }; template&lt;class T&gt; Base* Bar&lt;T&gt;::convert(ID id) const { if (id == FOO_FLOAT) return new Foo&lt;float&gt;(value); // Error. ... } </code></pre> <p>The error is, naturally, "invalid use of incomplete type".</p>
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    1. COCyclic dependencies are rarely a good idea. Try to refactor it so that the dependency is broken. A first idea would be moving the `convert` method into a free function that depends on both `Bar` and `Foo`...
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      1. USDavid Rodríguez - dribeas
 

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