C++ lambda with captures as a function pointer

Question

I was playing with C++ lambdas and their implicit conversion to function pointers. My starting example was using them as callback for the ftw function. This works as expecte

Answer 1

Using locally global (static) method it can be done as followed

template 
auto cify_no_args(F&& f) {
  static F fn = std::forward(f);
  return [] {
    return fn();
  };
}

Suppose we have

void some_c_func(void (*callback)());

So the usage will be

some_c_func(cify_no_args([&] {
  // code
}));

This works because each lambda has an unique signature so making it static is not a problem. Following is a generic wrapper with variadic number of arguments and any return type using the same method.

template 
struct lambda_traits : lambda_traits
{ };

template 
struct lambda_traits : lambda_traits
{ };

template 
struct lambda_traits {
    using pointer = typename std::add_pointer::type;

    static pointer cify(F&& f) {
        static F fn = std::forward(f);
        return [](Args... args) {
            return fn(std::forward(args)...);
        };
    }
};

template 
inline lambda_traits::pointer cify(F&& f) {
    return lambda_traits::cify(std::forward(f));
}

And similar usage

void some_c_func(int (*callback)(some_struct*, float));

some_c_func(cify([&](some_struct* s, float f) {
    // making use of "s" and "f"
    return 0;
}));