C++ std::function that returns a vector of it's own type (recursive types again)

Question

Ideally, I'd like to declare the following type:

using action_t = std::function< std::vector< action_t >(void) >

This is a thunk which returns a vector of followup thunks. I was able to get this far using information from Recursive typedef function definition : std::function returning its own type :

struct RecursiveHelper                                                                                                                                     
{                                                                                                                                                          
  typedef std::vector<RecursiveHelper> rtype;                                                                                                              
  typedef std::function< rtype (void) > ftype;                                                                                                             
  RecursiveHelper( ftype f ) : func(f) {}                                                                                                                  
  rtype operator()() const { return func(); }                                                                                                              
  operator ftype () { return func; }                                                                                                                       
  ftype func;                                                                                                                                              
};                                                                                                                                                         

using action_t = RecursiveHelper;                                                                                                                          
using actions_t = std::vector<RecursiveHelper>; 

However, to push these things onto a stack, for instance, I have to do things like this:

std::stack<action_t> stack;                                                                                                                            

stack.push(RecursiveHelper([&visitor, &node](void){                                                                                                    
    return visitor.visitNode(node);                                                                                                                    
    }));                                                                                                                                               

Ideally I'd like to avoid any mention of RecursiveHelper in code which uses this stuff, if they want a stack of action_t, they should be able to push conforming lambdas straight onto it.

Is there any way to accomplish this?

Answer 1

Write a constructor that accepts any function object that is convertible to ftype and isn't RecursiveHelper:

template<class F, class = std::enable_if_t<std::is_convertible<F, ftype>::value &&
                          !std::is_same<RecursiveHelper, std::decay_t<F>>::value>>
RecursiveHelper( F&& f ) : func(std::forward<F>(f)) {}

For C++11, replace something_t<...> with typename something<...>::type.