#ifndef __TEST__
#define __TEST__
namespace std
{
template<typename T>
class list;
}
template<typename T>
void Pop(std::list<T> * l)
{
while(!l->empty())
l->pop();
}
#endif
and used that function in my main. I get errors. Of course, I know that there are more template params for std::list (allocator I think). But, that is beside the point. Do I have to know the full template declaration of a template class to be able to forward declare it?
EDIT: I wasn't using a pointer before - it was a reference. I'll try it out with the pointer.
The problem is not that you can't forward-declare a template class. Yes, you do need to know all of the template parameters and their defaults to be able to forward-declare it correctly:
namespace std {
template<class T, class Allocator = std::allocator<T>>
class list;
}
But to make even such a forward declaration in namespace std is explicitly prohibited by the standard: the only thing you're allowed to put in std is a template specialisation, commonly std::less on a user-defined type. Someone else can cite the relevant text if necessary.
Just #include <list> and don't worry about it.
Oh, incidentally, any name containing double-underscores is reserved for use by the implementation, so you should use something like TEST_H instead of __TEST__. It's not going to generate a warning or an error, but if your program has a clash with an implementation-defined identifier, then it's not guaranteed to compile or run correctly: it's ill-formed. Also prohibited are names beginning with an underscore followed by a capital letter, among others. In general, don't start things with underscores unless you know what magic you're dealing with.
there is a limited alternative you can use
header:
class std_int_vector;
class A{
std_int_vector* vector;
public:
A();
virtual ~A();
};
cpp:
#include "header.h"
#include <vector>
class std_int_vector: public std::vectror<int> {}
A::A() : vector(new std_int_vector()) {}
[...]
not tested in real programs, so expect it to be non-perfect.
I solved that problem.
I was implementing an OSI Layer (slider window, Level 2) for a network simulation in C++ (Eclipse Juno). I had frames (template <class T>) and its states (state pattern, forward declaration).
The solution is as follows:
In the *.cpp file, you must include the Header file that you forward, i.e.
ifndef STATE_H_
#define STATE_H_
#include <stdlib.h>
#include "Frame.h"
template <class T>
class LinkFrame;
using namespace std;
template <class T>
class State {
protected:
LinkFrame<int> *myFrame;
}
Its cpp:
#include "State.h"
#include "Frame.h"
#include "LinkFrame.h"
template <class T>
bool State<T>::replace(Frame<T> *f){
And... another class.
Forward declaration should have complete template arguments list specified.
Source: Stackoverflow.com