Tranparently configure an std::function / lambda callback

Question

I have something like the following structure. It's a container which invokes a callback on all its elements. fct could be a lambda, a functor or a plain function pointer.

Now, in some cases I'd like to optionally configure fct before I call it. For example, fct computes a complicated function on each x. But ContainerX already knows an intermediate result - I would like to tell fct the intermediate result. If fct is a functor, it has a state and it can store the intermediate rsult. If it's a plain function, it can not store the intermediate result and should be called without the pre-configuration.

class ContainerX{ void callback(function<void(X* x)> fct){ // 1. Tell fct the intermediate result, if it is able to be configured // 2. Call fct for all x in the container } }

There are basically many ways to do this:

I could define fct as function<void(X* x, double result)>
I could define fct as an abstract class which has the actual function and a preconfiguration function.

But these solution require me to change the signature of callback in a way that I can no longer call it with plain functions as well as with functors.

Is there a way to transparently, optinally configure fct?

Let the functions have an optional parameter, that represents the intermediate result. — MikeMB
– MikeMB, Commented May 20, 2015 at 8:30
Sounds interesting, I was not aware of optional parameters in C++. Could you provide a short code snippet how this would look? (Particularly, the signature of fct. — Michael
– Michael, Commented May 20, 2015 at 8:36
What do you mean by "I can no longer call it with plain functions as well as with functors."? What would be the problem with having an overload? — laurisvr
– laurisvr, Commented May 20, 2015 at 8:45
I think a good C++ way should be to use virtual methods instead of callback. I solved a lot of problem using C++ virtual methods. — Sir Jo Black
– Sir Jo Black, Commented May 20, 2015 at 9:17
What I meant with optional parameters are e.g. a pointer to the intermediate result, that is null if no intermediate result can be provided or something like boost::optional. — MikeMB
– MikeMB, Commented May 20, 2015 at 9:39

MikeMB · Accepted Answer · 2015-05-20 09:40:34Z

I would create an overload of callback does use the result parameter. It would look something like this:

class ContainerX{ void callback(function<void(X* x)> fct){ } void callback(function<void(X* x, double result)>){ } }

But these solution require me to change the signature of callback in a way that I can no longer call it with plain functions as well as with functors.

There are in essence two different callback functions. One which takes a function using an intermediate value. One that doesn't. Sometime you want different signatures which do roughly the same thing. That's what overloads are for:).

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