[c-prog] Re: STL and forward declarations

[peternilsson42]

--- In c-prog@yahoogroups.com, xcianox2...@... wrote:
> Hi all,
> I tried to use STL for a C++ class that can contain
> arrays of itself as, for example, a directory tree.

Not sure what the little Â's are. I would say your email
client is inserting them (and other characters.)

> It is easy to implement this using normal pointers but
> I tried to use the std::vector container and got compile
> errors. The following is a code fragment:
>  
> #include <vector>
> #include <string>
> // forward declaration
> class DirectoryEntryArray;

Here you declare a class called DirectoryEntryArray.

> // describes a single directory entry: the name, the size,
> etc.. and
> // the type of the entry: it could be a subdirectory
> class DirectoryEntry
> {
> public:
>  std::string m_name;
>  int m_size;
>  std::string m_dateCreated;
>  int m_type; // 0=FILE, 1=LINK, 2=DIRECTORY, ...
>  // if m_type == DIRECTORY the entry contains an array of
> other entries
>  // we need a pointer because the array has an incomplete
> type
>  DirectoryEntryArray *m_array;
> };
> typedef std::vector<DirectoryEntry> DirectoryEntryArray;

Here you declare DirectoryEntryArray to be a synonym of another
class (std::vector<>).

Simpler is...

  typedef std::vector<class DirectoryEntry> DirectoryEntryArray;

  class DirectoryEntry
  {
    ...
    DirectoryEntryArray *m_array
  };

Even simpler is...

  class DirectoryEntry
  {
    ...
    std::vector<DirectoryEntry> *m_array;
  };

Even simpler still is...

  class DirectoryEntry
  {
    ...
    std::vector<DirectoryEntry> m_array;
  };

You don't really need pointers at this stage.

<snip>
> The code does not compile on my linux machine GCC 4.0.3.
> Here is the error:
>  
> test.cpp:27: error: conflicting declaration ‘typedef
> class std::vector<DirectoryEntry, std::allocator
> <DirectoryEntry> > DirectoryEntryArray’
> test.cpp:10: error: ‘struct DirectoryEntryArray’ has
> a previous declaration as ‘struct DirectoryEntryArray’

This actually tells you that you've defined the same name
for two different classes.

> ...I think the code is surely wrong but  do not undestand
> why is it wrong.

Because a forward declaration of a class name declares that
class. Attempting to then typedef it as a synonym for a
different class is not allowed.

What you've done is similar to...

  class A;  // declare a class called A
  class B;  // declare a class called B (think std::vector<>)
  typedef B A; // somehow redeclare class A to be class B?!

-- 
Peter