Pointers of template class in another class

[lorenzo17]

Hey everybody.
I am currently facing a problem with pointer of template class, and i cannot see any easy solution.

Here is the code :

// Agent.h
class Space;

template<class T> class Agent
{
public:
Agent(Space* a_space,...); // there are other variables here but not important for the code
virtual ~Agent();
.... (other member functions)
private:
Space* m_linkToSpace;
};
// Agent. cpp
#include "Space.h"
// declaration of Agent's functions
template class Agent<int>;
template class Agent<double>;

Then I have derived template agents from template class Agent.

Then all agents are in a space (or in a derived class from space)

// Space.h
template<class T> class Agent; // I tried also with #include "Agent.h"

class Space
{
public:
Space();
virtual ~Space();
... (other functions using pointers on Agent<T>)
private:
std::map<int,Agent<T>*> m_agents; // to store every agents in the space and their Ids.
};

// Space.cpp
#include "Agent.h"
// Declaration of Space's functions

This code does not compile because of Agent<T>* (it works when Space has no pointer on Agent<T>). I have tried different things (add some typename before Agent<T> or template<class T> friend class Agent or template Agent<int> at the end...) but none worked.

Does anyone see how to make the code works ?? THANK YOU !!



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[jsmith]

All of Agent.cpp has to be moved to Agent.h, first off.

Second, Space has to be a template class on T also.

[lorenzo17]

I tried to make Space.h a template on T but then I had some compiler problems when I included Space.h in Agent.h for the Space<T>* m_linktoSpace.

PS: I am sorry for the mess at the end of my previous post..

[jsmith]

Ugh, you are going to run into endless problems as a result of the circular reference. You're going to have to
rethink your design, otherwise you're going to have to explicitly instantiate Agent and/or Space on all the
types your program will use in their corresponding .cpp files.

[lorenzo17]

Thank you, I will try that now

[Disch]

Obligatory link: http://cplusplus.com/forum/articles/10627/ (specifically, section 7, maybe also section 8)

This isn't as big of a mess as jsmith suggested. You just have to know about #include dependencies and forward declared dependencies.

You won't be able to have .cpp files here. Instead you can have normal header files (I'd go with "whatever.h") and implementation header files (I typically go with "whatever.hpp")

Here's how you could do it:

//agent.h
#ifndef AGENT_H_INCLUDED
#define AGENT_H_INCLUDED

template <typename T> class Space;

template <typename T> class Agent
{
  // ... put your Agent class here
  //  do not put bodies for any functions which use 'Space' here
};

// then include the implementation file
#include "agent.hpp"

#endif // AGENT_H_INCLUDED 

space.h would look similar, only you'd forward declare Agent instead of Space, then #include "space.hpp" at the end.

agent.hpp and space.hpp would look like normal .cpp files:

// agent.hpp
//  you can put #include guards here if you want.  Although this file should not be included
//  by anything other than agent.h

#include "space.h"
#include "whatever other headers you need"

// give bodies to your functions here 

space.hpp would look the same.

Of course you could put everything in the .h file and skip the .hpp files, but I find that a bit messier. Just instead of putting things in a hpp file and #including it, you can just put it directly after the class body in the header.

[lorenzo17]

Thank you so much for your answer Disch!! Very interesting and worthwhile!!