Re: Forward template declaration problem

From:
"Bo Persson" <bop@gmb.dk>
Newsgroups:
comp.lang.c++
Date:
Sat, 12 Mar 2011 11:54:38 +0100
Message-ID:
<8u11ndFhb6U1@mid.individual.net>
Jure Erznoznik wrote:

I'd like to hide implementation details from declaration because
dragging all the support types from .cpp to .h would be unnecessary,
redundant and just plain ugly.
So I try to forward-declare the classes used in actual
implementation. This works well for ordinary classes, but not so
well for templates (std::multimap in this case).

I know the typedef from the following sample redeclares my desired
type, but I included the code because I don't know any better. I
also included a sample for MyClass class which compiles fine - for
reference of what I want to do.

So how can I make the below code work without:
1. Dragging all the necessary types into .h
2. Declaring jobList member as void * and then typecasting all over
the place
3. Using #define to mask the typecasting from #2

Please forgive the fact that I'm still learning C++.

Thanks,
Jure

//sample.h here
class JobListMap;
class MyClass;

class RefClock
{
public:
 RefClock();
 JobListMap *jobList;
 MyClass *test;
};

//sample.cpp from now on
#include "sample.h"
#include <map>

struct MyType1 {
...
};
struct MyType2 {
...
};
class MyClass {
};

typedef std::multimap<MyType1, MyType2> JobListMap;


No way! :-)

Earlier you said that JobListMap is a class, now you say it's a
typedef. Can't do that!

A possible, but ugly, way of actually making it a class is

class JobListMap : public std::multimap<MyType1, MyType2>
{
    // whatever constructors you need here
};

RefClock::RefClock()
{
 jobList = new JobListMap();
 test = new MyClass();
}


Bo Persson

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