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Forward template declaration problem

 
 
Jure Erznožnik
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      03-12-2011
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;

RefClock::RefClock()
{
jobList = new JobListMap();
test = new MyClass();
}
 
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Bo Persson
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      03-12-2011
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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Jure Erznožnik
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Posts: n/a
 
      03-12-2011
Tried declaring that, but the compiler doesn't like me:

class JobListMap: public std::multimap<MyType1, MyType2>
{
public:
JobListMap(): std::multimap<MyType1, MyType2>() {}
};

I get the folowing errors:
refclock.cpp(89) : error C2512: 'bitlib::JobListMap' : no appropriate
default constructor available
refclock.cpp(100) : warning C4150: deletion of pointer to incomplete
type 'bitlib::JobListMap'; no destructor called
include\blaf/refclock.h(33) : see declaration of
'bitlib::JobListMap'
refclock.cpp(10 : error C2027: use of undefined type
'bitlib::JobListMap'
include\blaf/refclock.h(33) : see declaration of
'bitlib::JobListMap'
refclock.cpp(10 : error C2227: left of '->insert' must point to
class/struct/union/generic type

So, is there a non-ugly way of doing this?
 
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Victor Bazarov
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      03-12-2011
On 3/12/2011 6:15 AM, Jure Erznonik wrote:
> Tried declaring that, but the compiler doesn't like me:
>
> class JobListMap: public std::multimap<MyType1, MyType2>
> {
> public:
> JobListMap(): std::multimap<MyType1, MyType2>() {}
> };
>
> I get the folowing errors:
> refclock.cpp(89) : error C2512: 'bitlib::JobListMap' : no appropriate
> default constructor available


Did you forget to include the proper header where your 'JobListMap'
class is defined? This error is likely to occur when the compiler can
only see the forward declaration.

> refclock.cpp(100) : warning C4150: deletion of pointer to incomplete
> type 'bitlib::JobListMap'; no destructor called
> include\blaf/refclock.h(33) : see declaration of
> 'bitlib::JobListMap'
> refclock.cpp(10 : error C2027: use of undefined type
> 'bitlib::JobListMap'
> include\blaf/refclock.h(33) : see declaration of
> 'bitlib::JobListMap'
> refclock.cpp(10 : error C2227: left of '->insert' must point to
> class/struct/union/generic type
>
> So, is there a non-ugly way of doing this?


You haven't done enough yet to claim ugliness. Get rid of the compile
errors first, make it work. Then you can see whether it's nice or ugly.

V
--
I do not respond to top-posted replies, please don't ask
 
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Jure Erznonik
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Posts: n/a
 
      03-12-2011
On Mar 12, 3:31*pm, Victor Bazarov <(E-Mail Removed)> wrote:
> On 3/12/2011 6:15 AM, Jure Erzno nik wrote:
>
> > Tried declaring that, but the compiler doesn't like me:

>
> > class JobListMap: public std::multimap<MyType1, MyType2>
> > {
> > public:
> > * *JobListMap(): std::multimap<MyType1, MyType2>() {}
> > };

>
> > I get the folowing errors:
> > refclock.cpp(89) : error C2512: 'bitlib::JobListMap' : no appropriate
> > default constructor available

>
> Did you forget to include the proper header where your 'JobListMap'
> class is defined? *This error is likely to occur when the compiler can
> only see the forward declaration.
>
> > refclock.cpp(100) : warning C4150: deletion of pointer to incomplete
> > type 'bitlib::JobListMap'; no destructor called
> > * * * * *include\blaf/refclock.h(33) : see declaration of
> > 'bitlib::JobListMap'
> > refclock.cpp(10 : error C2027: use of undefined type
> > 'bitlib::JobListMap'
> > * * * * *include\blaf/refclock.h(33) : see declaration of
> > 'bitlib::JobListMap'
> > refclock.cpp(10 : error C2227: left of '->insert' must point to
> > class/struct/union/generic type

>
> > So, is there a non-ugly way of doing this?

>
> You haven't done enough yet to claim ugliness. *Get rid of the compile
> errors first, make it work. *Then you can see whether it's nice or ugly..
>
> V
> --
> I do not respond to top-posted replies, please don't ask


Turns out I messed up with namespaces.
My header was in my_named_space.
In the source file I had
using my_name_space;

but the
class MyType ......

was in default namespace

fixed with class my_name_space::MyType

....

Thanks for all the help,
Jure
 
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