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    <p>This is not really the best way to do this, either from a pure OO point of view or a functional or procedural POV. First of all, your class A is really nothing more than a namespace that has to be instantiated. Personally, I'd just put its functions as free floating C-style ones - maybe in a namespace somewhere so that you get some kind of classification.</p> <p>Here's how you'd do it in pure OO:</p> <pre><code>class Function { virtual double Execute(double value); }; class Function1 : public Function { virtual double Execute(double value) { ... } }; class FunctionProcessor { void Process(Function &amp; f) { ... } } </code></pre> <p>This way, you could instantiate <code>Function1</code> and <code>FunctionProcessor</code> and send the Function1 object to the Process method. You could derive anything from <code>Function</code> and pass it to Process.</p> <p>A similar, but more generic way to do it is to use templates:</p> <pre><code>template &lt;class T&gt; class FunctionProcessor { void Process() { T &amp; function; ... } } </code></pre> <p>You can pass anything at all as T, but in this case, T becomes a compile-time dependency, so you have to pass it in code. No dynamic stuff allowed here!</p> <p>Here's another templated mechanism, this time using simple functions instead of classes:</p> <pre><code>template &lt;class T&gt; void Process(T &amp; function) { ... double v1 = function(x1); double v2 = function(x2); ... } </code></pre> <p>You can call this thing like this:</p> <pre><code>double function1(double val) { return blah; } struct function2 { double operator()(double val) { return blah; } }; // somewhere else FunctionProcessor(function1); FunctionProcessor(function2()); </code></pre> <p>You can use this approach with anything that can be called with the right signature; simple functions, static methods in classes, functors (like <code>struct function2</code> above), <code>std::mem_fun</code> objects, new-fangled c++11 lambdas,... And if you use functors, you can pass them parameters in the constructor, just like any object.</p> <p>That last is probably what I'd do; it's the fastest, if you know what you're calling at compile time, and the simplest while reading the client code. If it has to be extremely loosely coupled for some reason, I'd go with the first class-based approach. I personally think that circumstance is quite rare, especially as you describe the problem.</p> <p>If you still want to use your <code>class A</code>, make all the functions static if they don't need member access. Otherwise, look at <code>std::mem_fun</code>. I still discourage this approach.</p>
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    1. POWhat is a good design to use external class on member functions?
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    1. COthis is interesting. class A is really a namespace that has to be instantiated, but I have other classes that share some behaviour with class A so I was hoping I could derive them all from a base class. therefore I might choose your first example and make Function_processor work with the base type (not exactly what I wanted but probably ok). your last example I can not use unless I give up on the concept that A is a class.
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    2. COYou can use the third one with a class - just overload `operator()` like I did with the struct. In C++, the only difference between a class and a struct is that struct members are public by default. You can also look into `std::mem_fun` or one of the bind family of functions to create something to pass to FunctionProcessor. It can remain a class and you can bind to its member functions.
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    3. COthe problem with operator() is that A has multiple functions with the same signature. I think I will use FastDelegate now.
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