Re: alternative for virtual template function?

martinb <>
Wed, 23 Jul 2008 07:10:18 -0700 (PDT)

I have an abstract base class which contains a function that I would
like to template, but virtual template functions are illegal. I put a
mini code example below, which doesn't do anything great, but reflects
the structure of my problem.

In a nutshell, the abstr. base class VectorCreator has a function
createVector whose return type is unfortunately fixed to
vector<double>. Only now I discovered that some implementations would
like to work with other types, such as vector<int>. I could just
template the VectorCreator class, but the problem is that pointers to
VectorCreator are passed around in the whole project, so templating it
would require too many changes. It's okay to change the createVector
function, but not the whole VectorCreator class (by introducing a
class-scope template).

I think you'd better template the class. The problem with old code can
circumvent=C5d by introducing a new class template and typdef-ing it:

template<typename T>
class VectorCreatorTempl {
  virtual ~VectorCreator() {}
  virtual std::vector<T>* createVector() = 0;

typedef VectorCreatorTempl<double> VectorCreator;

BTW: For factory functions it is generally preferred to return an
instead of a raw pointer.

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