Recursive std::function definition inside template class method

Question

For the template class declarations:

enum class traversal_order {PREORDER, INORDER, POSTORDER};

template<typename T1>
class node{

//public and private members


};

template<typename T1, typename T2>
class tree{

//rest of template
public:
using sp2node_cont = vector<shared_ptr<node<T1>>>;
using r_iter_sp2node_cont = typename vector<shared_ptr<node<T1>>>::iterator&;

public:

tree(T2 const&, size_t);
void traversal_tree(traversal_order);

//rest of template

private:
sp2node_cont impl_tree;

};

And for the following definition of the template method:

template<typename T1, typename T2>
tree<T1,T2>::traverse_tree(traversal_order D){

switch(D){

   case PREORDER:
   {
     function<void(r_iter_sp2node_cont,r_iter_sp2node_cont)>
     print_preorder = [&] (r_iter_sp2node_cont begin,
                           r_iter_sp2node_cont end
                          ){

             //rest of the definition

            print_preorder(std::next(begin), end);

     };

     print_preoder(impl_tree.begin(), impl_tree.end());

   break;


    }


  }

}

The compiler generates the following error for the recursive invocation of the std::function print_preorder() when invoked from main(), after constructing the tree object as follows:

std::array<string,25> A;
//rest of the code
tree<string, decltype(A)> a_tree(A,A.size());
a_tree.traverse_tree(traversal_order::PREORDER);




   1 In file included from main.cpp:1:
   2 ./bt.hpp:306:21: error: no matching function for call to object of type         'function<void (r_iter_sp2node_cont, r_iter_sp2node_cont)>' (aka                'function<void (__wrap_iter<std::__1::shared_ptr<node<std::__1::                basic_string<char> > > *> &, __wrap_iter<std::__1::shared_ptr<node<std::        __1::basic_string<char> > > *> &)>')
   3                     print_preorder(next(begin), end);
   4                     ^~~~~~~~~~~~~~ 
   5 main.cpp:23:12: note: in instantiation of member function 'tree<std::__1::      basic_string<char>, std::__1::array<std::__1::basic_string<char>, 25> >::       traverse_tree' requested here 
   6     a_tree.traverse_tree(traversal_order::PREORDER);
   7            ^
   8 /Library/Developer/CommandLineTools/usr/include/c++/v1/functional:1667:9:       note: candidate function not viable: expects an l-value for 1st argument
   9     _Rp operator()(_ArgTypes...) const;
  10         ^       
  11 1 error generated.

I am not sure how to resolve this error. Any suggestions?

Answer 1

Your issue is in this line

using r_iter_sp2node_cont = typename vector<shared_ptr<node<T1>>>::iterator&;

you have defined r_iter_sp2node_cont as reference to iterator.

So the signature of print_preorder is

print_preorder(vector...::iterator&, vector...::iterator&)

But when you call

print_preorder(impl_tree.begin(), impl_tree.end());

methods of vector begin() and end() returns iterator objects. You cannot pass temporary iterator object returned by these member functions to function which expects reference to iterator iterator& .

You should remove & from definition r_iter_sp2node_cont type.

PS. Your code can be compiled under MSVC which has extension and temporary object can be passed to function with expects reference to object void foo(anObject&) .