In this post, we’ll take a closer look at how to extend the earlier callback wrapper mechanism to handle regular function pointers as well as member functions. Along the way, we’ll examine some of the subtleties of inferring function pointer types from callable objects—especially when lambdas with auto parameters enter the picture.

The problem with inferring from a function call operator is that there may be more than one

by Raymond Chen

From the article:

Some time ago, I wrote briefly on writing a helper class for generating a particular category of C callback wrappers around C++ methods. This particular mechanism used a proxy object with a templated conversion operator to figure out what function pointer type it was being asked to produce.¹

But what about taking a std::invoke‘able object and inferring the function pointer from the parameters that the invoke‘able’s operator() accepts?

Sure, you could try to do that, but there’s a catch: There might be more than one operator().

The common case of this is a lambda with auto parameters.

RegisterCallback(
    CallableWrapper([](auto first, auto second, auto third) {
        ⟦ ... ⟧
    }, context);