c++ nested class protyping

c++ nested class protyping

oscar_spencer

Member #8,914

August 2007

How does one prototype a nested class?

for example with this in one file

class Outer
{
public:
  class Inner
  {
    ...
  };
  ...
};

i prototype Outer in another file.

class Outer;

but how do i prototype Inner?
Ive tried

class Outer::Inner;

but it gives me errors when i declare a pointer of type Inner later

Outer::Inner * variable;

Any help much appreciated.
--scott

Roy Underthump

Member #10,398

November 2008

http://www.allegro.cc/forums/thread/594939

I'm not sure if it's quite the same problem

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oscar_spencer

Member #8,914

August 2007

thanks, but not quite the same problem
AFAIK namespaces allow multiple entries in a file
like this is legal

namespace ANamespace
{
  ...definitions
}
...other code
namespace ANamespace
{
  ...definitions
}

this is not legal

class AClass
{
  ...definitions
};
...other code
class AClass
{
  ...definitions
};

anonymous

Member #8025

November 2006

You might just make Inner a non-nested normal class. If it is inner for name control only (so that the name of Outer would be part of the class name), perhaps a namespace could be used instead.

As far as I know, there isn't anything really special about nested classes, except perhaps conveying the meaning that they should only be used by the enclosing class (when declared private), so that there are no objective reasons that would force you to use one or another.

Thomas Fjellstrom

Member #476

June 2000

I had the same issue with a "little" C++ lib I was working on. Make the inner class a regular class with a funny name, and typedef it into the outer class if you still want to access it inside like you did before.

but to be brutally honest, nested classes are useless in C++. They really don't provide anything usefull over a regular class.

--
Thomas Fjellstrom - [website] - [email] - [Allegro Wiki] - [Allegro TODO]
"If you can't think of a better solution, don't try to make a better solution." -- weapon_S
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Kitty Cat

Member #2,815

October 2002

Quote:

but to be brutally honest, nested classes are useless in C++. They really don't provide anything usefull over a regular class.

It does make it so the nested class can only be used within the proper scope of the container class. If you have a nested class inside a protected/private section, nothing outside of the container class (and maybe it's derivitives) that isn't specificly friended can use it.

--
"Do not meddle in the affairs of cats, for they are subtle and will pee on your computer." -- Bruce Graham

ReyBrujo

Moderator

January 2001

Does that work? Never tried nested classes, makes everything harder to read in headers.

--
RB
光子「あたしただ…奪う側に回ろうと思っただけよ」
Mitsuko's last words, Battle Royale

Paul whoknows

Member #5,081

September 2004

1class CMyClass
2{
3  private:
4  
5  public:
6    CMyClass();
7  
8  class CMyOtherClass
9  {
10    private:
11    public:
12      CMyOtherClass();
13  
14  };
15  
16}
17 
18CMyClass::CMyClass()
19{
20  stuff here
21}
22 
23CMyClass::CMyOtherClass::CMyOtherClass()
24{
25  stuff here
26}
27 
28//Hope you get the idea.

____

"The unlimited potential has been replaced by the concrete reality of what I programmed today." - Jordan Mechner.

ReyBrujo

Moderator

January 2001

Yuck, awful. Looks like nested namespaces.

--
RB
光子「あたしただ…奪う側に回ろうと思っただけよ」
Mitsuko's last words, Battle Royale

anonymous

Member #8025

November 2006

The nested class should be in the private/protected section of the outer class. Then it is available only for the containing class and you'll get a compiler error should you try to use the nested class elsewhere ("class X is private in this context").

Of course, you could also use separate classes and make everything in the "nested" class private and give access to the "outer" class through friendship, only that gives "outer" access to everything, not just its public interface.

Paul whoknows

Member #5,081

September 2004

Quote:

Yuck, awful.

I find inner classes useful when I have to implement relatively complex tasks in the methods of the outer class.
If you need classes for your program, I can't see why you shouldn't need classes for your methods?

____

"The unlimited potential has been replaced by the concrete reality of what I programmed today." - Jordan Mechner.

Thomas Fjellstrom

Member #476

June 2000

Quote:

I find inner classes useful when I have to implement relatively complex tasks in the methods of the outer class.

But they really don't need to be inner classes then, just a small helper class, doesn't really matter where you put it.

oscar_spencer

Member #8,914

August 2007

Thanks for all the replies,

seems the general consensus is to not use nested classes
unless theyre private.
for my use it should be possible to use a helper class
instead.

I think that it makes the interface cleaner tho, especially
like in the STL.

list<Object>::iterator it;

anonymous

Member #8025

November 2006

Don't forget about the typedefs. I'm pretty sure that in your STL implementation you'll find that the iterators are not nested types and the "nestedness" comes from typedefs like this:

      typedef _List_iterator<_Tp>                        iterator;
      typedef _List_const_iterator<_Tp>                  const_iterator;
      typedef std::reverse_iterator<const_iterator>      const_reverse_iterator;
      typedef std::reverse_iterator<iterator>            reverse_iterator;

As you can see, the STL already contains an implementation of std::reverse_iterator that can be used to synthesize reverse iterators from any bidirectional iterator, so at least this couldn't be a nested class.