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    <blockquote> <p>I don't know in advance I am going to have a Blah, only a Blah.</p> </blockquote> <p>From the language point of view, <code>Blah&lt;T&gt;</code> is meaningless because <code>T</code> doesn't exist. Depending on what you're exactly trying to do, you can</p> <p>make <code>Foo</code> a template, too, so that you can declare a template parameter <code>T</code>:</p> <pre><code>template&lt;typename T&gt; class Foo { public: Foo getInstance(const string&amp; fooType); ... private: shared_ptr&lt; FooImpl&lt;T&gt; &gt; m_impl; }; </code></pre> <p>which 'fixes' the choice of <code>T</code> when you declare a variable of type <code>Foo&lt;T&gt;</code>;</p> <p>or make <code>FooImpl</code> explicitly derive from a common base:</p> <pre><code>class FooBase { // need to define the interface here }; // this is a class definition whereas previously you only needed a declaration template&lt;typename T&gt; class FooImpl: public FooBase { // definition here }; class Foo { public: Foo getInstance(const string&amp; fooType); // we needed the definition of FooImpl for this member // in addition this member is quite obviously a template template&lt;typename T&gt; void set(FooImpl&lt;T&gt; const&amp; foo) { m_impl.reset(new FooImpl&lt;T&gt;(foo)); } // not a member template! void use() { // any use of m_impl will be through the FooBase interface } private: shared_ptr&lt;FooBase&gt; m_impl; }; </code></pre> <p>where for a given <code>Foo</code> instance any kind of <code>FooImpl&lt;T&gt;</code> can be set dynamically and then used through the <code>FooBase</code> interface. This is a kind of <a href="https://stackoverflow.com/questions/5450159/type-erasure-techniques">type erasure</a> as it's called in the C++ world.</p>
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