Question

I am trying to write a function that takes two containers of the same contained type, e.g., two std::vector<int>s, or a std::list<int> and a std::vector<int>. (But not a std::vector<int> and a std::vector<double>!)

Since I am not quite sure regarding how it should be done, I decided to write a test progam first:

#include <iostream>
#include <vector>
#include <list>
#include <algorithm>

struct vector_wrapper
{
  template <typename T>
  struct instance_wrapper
  {
    typedef typename std::vector<T> instance;
  };
};

struct list_wrapper
{
  template <typename T>
  struct instance_wrapper
  {
    typedef typename std::list<T> instance;
  };
};

template <typename T, typename C1, typename C2>
void move(typename C1::instance_wrapper<T>::instance& c1, typename C2::instance_wrapper<T>::instance& c2) // line 29
{
  while (c1.size() > 0)
  {
    c2.push_front(c1.back());
    c1.pop_back();
  }
}

int main()
{
  std::vector<int> v;
  std::list  <int> l;

  v.reserve(10);
  for (int i = 0; i < 10; ++i)
    v.push_back(i);

  move<int, vector_wrapper, list_wrapper>(v, l);

  std::for_each(l.begin(), l.end(),
    [] (int i) { std::cout << i << " "; }
  );

  std::cout << std::endl;

  return 0;
}

---

This code gives me the following compile-time error with g++ 4.7, using the -std=c++11 flag:

metaclass.cpp:29:24: error: non-template 'instance_wrapper' used as template
... more ...

Why does the compiler not correctly identify instance_wrapper as a template?

Answer 1

Compiler has already told you what is wrong (errors from ideone):

prog.cpp:25:24: error: non-template ‘instance_wrapper’ used as template
prog.cpp:25:24: note: use ‘C1::template instance_wrapper’ to indicate that it is a template

Use C1::template instance_wrapper instead of C1::instance_wrapper – and, similarly, do the same for C2::instance_wrapper:

template <typename T, typename C1, typename C2>
void move(typename C1::template instance_wrapper<T>::instance& c1, 
    typename C2::template instance_wrapper<T>::instance& c2)
{
    // ...

It’s because C1 is template and compiler cannot deduce that instance_wrapper is template and treats it as non-template type.

Please, please, read everything compiler outputs. Not just line-by-line. Often compiler says what’s wrong in one of previous or following lines, as in this case, when it’s already giving you the answer!