Make variable class member outside of constructor - c++

Question

Basically, I need to set a variable outside of the constructor and make it accessible to the entire class.
It would need to work something like this:
#include <iostream>
#include <string>
template <typename MT>
class CallbackFunction
{
void (*func)(MT);
MT *data;
public:
void SetCallbackData (void (*f)(MT), MT *d)
{
func = f;
data = d;
}
void Call()
{
func(data);
}
};
class Callback
{
public:
template <typename T>
void SetCallback(CallbackFunction <T> *func)
{
// Need to make this a class member;
CallbackFunction <T> *CallbackClass = func;
}
void Call()
{
CallbackClass->Call();
}
};
template <typename CT>
Callback *NewCallback(void (*func)(CT), CT *data)
{
Callback *cb;
CallbackFunction <CT> *cf;
cf->SetCallbackData(func, data);
cb->SetCallback <CT> (cf);
return cb;
};
void Call(Callback *CallbackFunc)
{
CallbackFunc->Call();
}
void foo(std::string str)
{
std::cout << str << "\n";
}
int main()
{
std::string *str;
str->append("Hello, World!");
Call( NewCallback(foo, str) );
return 0;
}
More details:
I know it's buggy, and it doesn't compile, I'll sort out those bugs when I find a solution to my problem. Which is:
I need to find a way to declare a template variable inside a member function of the class "Callback". I need to do this because the class "Callback" cannot be a template, it needs to remain a simple class. So because the class "Callback" is not a template, I need to make one of it's member functions a template instead. So that member function can declare a variable of the type defined (with the template) when the function is called, and this variable needs to be accessible to the entire class.
So in a nice list:
class "Callback" cannot be a template,
variable CallbackClass must be accessible to the entire class,
but remain inside of the class.

Answer 1

#include <iostream>
#include <string>
#include <memory>
template <typename MT>
class CallbackFunction
{
typedef void (*func_ptr)(MT);
func_ptr f_ptr;
typedef std::shared_ptr<MT> data_ptr;
data_ptr data_p;
public:
void SetCallbackData (func_ptr f_ptr_, MT *d)
{
f_ptr = f_ptr_;
data_p.reset(d);
}
void Call()
{
if ( f_ptr ) f_ptr(data);
}
};
template<class T>
class Callback
{
public:
template <typename T>
void SetCallback(CallbackFunction <T> *func)
{
f_ptr.reset(func);
}
void Call()
{
if ( f_ptr ) f_ptr->Call();
}
typedef std::shared_ptr<CallbackFunction<T>> func_ptr;
static func_ptr f_ptr;
};

Answer 2

I would implement this using polymorphism. Your programming skills seem good so I will just sketch the direction to solution, feel free to ask for more help if needed.
// your callbackobjects inherit from this class, the sole purpose of this
// class is to provide the Call interface. The derived classes implement
// their custom version of Call().
class CallBackObject{
public:
virtual void Call(){};
};
class Callback
{
CallBackObject *callBackObject;
public:
void SetCallback(CallBackObject *o)
{
callBackObject = o;
}
void Call()
{
callBackObject -> Call();
}
};

Answer 3

Create an abstract interface Callback class and have your CallbackFunction<T> inherit from this. Have your Callback class hold a pointer to this abstract interface. Finally, have your Callback::SetCallback assign func to this pointer.
Here's some code to illustrate the idea:
class ICallback
{
public:
virtual ~ICallback() {}
virtual void Call() = 0;
};
template <typename MT>
class CallbackFunction : public ICallback
{
typedef void (*callback)(MT);
callback myfunc;
MT *data;
public:
CallbackFunction (callback f, MT *d) :
myfunc (f),
data (d)
{}
void Call()
{
if(myfunc && data)
{
myfunc(*data);
}
else throw std::logic_error("Callback function or data is null!");
}
};
Then have Callback hold a ICallback*:
class Callback
{
ICallback *mycallback;
public:
template <typename T>
void SetCallback(CallbackFunction <T> *func)
{
// Need to make this a class member;
// CallbackFunction <T> *CallbackClass = func;
mycallback = func;
}
void Call()
{
mycallback->Call();
}
};
The idea is to make all instantiated templates of CallbackFunction <T> a kind-of ICallback. Now the class using ICallback can take any class CallbackFunction <T> without needing to know what T is.