Question

Given a template class as such:

template <typename TYPE>
class SomeClass {
 public:
  typedef boost::intrusive_ptr<SomeClass<TYPE> > Client_t;
  inline Client_t GetClient() { return Client_t(this); }
};

SomeClass is intended only to be used via pointer references returned by SomeClass::GetClient(). Which makes it natural to write a wrapper function around creation like this:

template <typename TYPE>
SomeClass<TYPE>::Client_t New_SomeClass() { 
  return (new SomeClass<TYPE>)->GetClient(); 
}

Compiling the above code under GCC 4.4:

SomeClass<int>::Client_t some_class = New_SomeClass();

Gives the error “‘New_SomeClass’ was not declared in this scope”

Now I’m no template wizard, so there could be details here I’m not aware of, but I’m guessing I can’t use a construct of this sort at all due to the fact that C++ doesn’t allow overloading on return type.

I guess a…shiver… macro would solve it:

#define NEW_SOMECLASS(TYPE) ((new SomeClass<TYPE>)->GetClient())

auto some_class = NEW_SOMECLASS(int);

But there has to be a sensible way to expose object creation of a template class without resorting to macros or other cumbersome constructs?

Answer 1

SomeClass<int>::Client_t some_class = New_SomeClass<int>();

Because template parameters for New_SomeClass don’t depend on a function parameter, you must specify them. The error message you reported is a little strange for this problem, however, so you might have something else going on.

Or, my preference instead of New_SomeClass function:

template<class T>
struct SomeClass {
  typedef boost::intrusive_ptr<SomeClass> Client;
  inline Client client() { return Client_t(this); }

  static Client create() { return (new SomeClass())->client(); }

private:
  SomeClass(); // can be public too, if you really need it accessible
};

//...
SomeClass<int>::Client some_class = SomeClass<int>::create();

Even though it essentially just moves the function “into” the class, I find it generally cleaner.

In any case, if your compiler supports 0x’s ‘auto’ then you can use it:

auto some_class = SomeClass<int>::create();