Re: Templates functions as template parameters

From:

John Gilson <jag@acm.org>

Newsgroups:

comp.lang.c++

Date:

Tue, 26 Feb 2008 02:19:26 -0800 (PST)

Message-ID:

<952e42eb-0820-45eb-a63c-aaff843e2a5e@u10g2000prn.googlegroups.com>

On Feb 25, 1:42 pm, tygro <hubrys.neme...@gmail.com> wrote:

Hi,
I would like to do something like this:

class Type
{
public:

  template <template<typename> class Fun>
  void applyOnType(Fun f, void* data)
  {
   switch(type) {
    case 0:
      f<int>(data);
      break;
    case 1:
      f<short>(data);
      break;
   }

};

Then I could call:
template <typename T> void myFunction(T* data) { }

DataType dt;
dt.applyOnType(myFunction, data);

But it doesn't work because I guess it is not possible to pass a
template function in argument to a function.
Furthermore, because of the template-template, it seems that Fun must be
functor.
Do you see a way to write something as flexible as this but which could
compile?

My only working solution was this:
class TypeFunc
{
public:
   template<typename T> void operator()() {}

};

void onDataType(TypeFunc& f, void* data)
{
   switch(..) {
    ...
   f.operator()<int>();
  ...
  }

}

I don't think it's easy to use. I also have a macro based solution, but
I would prefer something 100% template.

Thank you for your help.

You're on the right track. You can't pass a function template as an
argument (a function template is a template but not a function) but
you can pass an object whose type is a class that has defined an
appropriate member function template:

template <typename T> void myFunction(T * data);

class TypeFunc
{
public:
  template<typename T> void operator()(T * data) const
  {
    myFunction(data);
  }
};

class Type
{
public:
  template<typename F>
  void applyOnType(const F & f, void * data)
  {
    switch(type) // Assume "type" is defined in scope
    {
    case 0:
      f(static_cast<int *>(data));
      break;
    case 1:
      f(static_cast<short *>(data));
      break;
    // ...
    }
  }
};

Type t;
void * data = ...;
t.applyOnType(TypeFunc(), data);

-- JAG