Re: template-nested STL iterator question

From:
"Alf P. Steinbach" <alfps@start.no>
Newsgroups:
comp.lang.c++
Date:
Wed, 10 May 2006 21:29:51 +0200
Message-ID:
<4cet9kF15i6bfU1@individual.net>
* chriscorbell:

I'm curious about what appears to be a restriction on using an STL
container inside a user-defined template, esp. using an iterator to
such a container. It's not clear to me if this is a general
template/language restriction, and STL iterator limitation, or if I'm
just going about it wrong.

I'm declaring a template which uses a std::map to store references to
the template type, e.g.

template template <typename T>
class MyClass
{
public:
  // ...
private:
  std::map<std::string, T*> m_objectMap;
};

This compiles fine.


I get a syntax error on the first line.

Which compiler is it that accepts the above?

However if I have a declaration of an iterator to
this map in a method of my class, the compile fails, e.g.

template template <typename T>
class MyClass
{
public:
  T* Lookup(std::string)
  {
      T* pT = NULL;
      std::map<std::string, T*>::iterator iTptr = m_objectMap.find();
      if(iTptr != m_objectMap.end())
         pT = *iTptr;
      return pT;
  }
private:
  std::map<std::string, T*> m_objectMap;
};


Curiously, as long as the Lookup function isn't actualy called, with the
first line corrected this (incorrectly) compiles fine with MSVC 7.1.

Is there a correct way to accomplish this?


Presumably you intended to (1) pass that string argument as 'std::string
const& s', (2) supply some argument to 'find', e.g. '.find(s)', and (3)
access the second field of the found pair, 'pT = iTptr->second'.

If not, anyone know the rationale?


For what?

The kludgy workaround I'm using is to use void * instead of T* for the
std::map and static-cast it.


That shouldn't work.

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