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    <p>I don't think it is possible to invoke a member of a class that is not defined in the current scope. The functor in this case retains no state and behaves like a function that takes no parameters and returns no value, so you could assign an instance of it to a raw function pointer. However, it is then no longer a <em>foo</em>: it is just a function pointer.</p> <p>One solution is to derive <em>foo</em> from a pure virtual base class defined at global scope, such as:</p> <pre><code>class base { public: void operator()() { doit(); } protected: virtual void doit() = 0; }; int main() { class foo { public: void operator()() { doit(); } protected: virtual void doit() { std::cout &lt;&lt; "Hello" &lt;&lt; std::endl; } }; } </code></pre> <p>You can now pass an instance of <em>foo</em> around as <em>base</em>, and the virtual doit() method will be invoked as expected.</p>
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    1. CO"I don't think it is possible to invoke a member of a class that is not defined in the current scope." That's nonsense. Whenever function objects are you rarely ever call a member function of a class that's defined in the current scope. (If you pass a function object to `std::for_each()`, it's `operator()` is called in of that function, but it's certainly not defined in its scope.)
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    2. CO@sbi - The problem is not with directly calling the operator(), but with calling the foreach. If you have: void MyFunc() {< Define class foo here >} Then you can't refer to 'foo' outside of MyFunc. You have to send it somehow (but of course you know that already). I think using inheritance is a legitimate solution, so no need to vote it down. The "not defined in the current scope" can be rephrased to "doesn't have access from the current scope", in which case it should be transferred from within MyFunc (or main) as a value, either by boost::function of by another means with inheritance.
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    3. COFair enough; I wasn't sufficiently careful in my wording. My point is that C++ doesn't support true runtime binding, a consequence of which is that you can't invoke something that is completely unknown. There is nothing magic about functors. foo is completely unknown outside of main(). Any solution, be it inheritance or whatever, has to make the essence of foo (its public executable interface) accessible outside of main(), and it has to do so at compile time. COM is perhaps the most elaborate example.
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