FSM functions (void*)

function_returns_self_type_example.cpp

#include <iostream>

struct FuncPtr;
typedef FuncPtr(Func)(int);

struct FuncPtr {
  void *func;
  FuncPtr(Func& f) : func{(void *) &f} {};
  FuncPtr operator()(int x) { return ((Func *)func)(x); }
};

// return self:
FuncPtr func_foo(int x) {
  std::cout << "func_foo " << x << "\n";
  return func_foo;
}

// return each other (like FSM states):
// error: illegal initializer (only variables can be initialized)
// Func func_baz = nullptr;
Func func_baz;

FuncPtr func_bar(int x) {
  std::cout << "func_bar " << x << "\n";
  //return {nullptr};
  // error: no matching constructor for initialization of 'FuncPtr'
  return func_baz;
}

FuncPtr func_baz(int x) {
  std::cout << "func_baz " << x << "\n";
  return func_bar;
}

int main() {
  func_foo(3)(5)(7);
  std::cout << "\n";
  func_bar(3)(5)(7);
  //FuncPtr null{nullptr};
  // error: no matching constructor for initialization of 'FuncPtr'
  //null(4);
  return 0;
}

$ clang++ -std=c++11 -Wall -Wconversion function_returns_self_type_example.cpp 
$ ./a.out 
func_foo 3
func_foo 5
func_foo 7

func_bar 3
func_baz 5
func_bar 7

Godbolt link.