Tranparently configure an std::function / lambda callback

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?

Source: c++

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