Re: template template parameters (was Re: [boost] Re: Special functions, octonions, quaternions in the CVS)

From: Jeremy Siek (jsiek_at_[hidden])
Date: 2001-06-26 10:32:38

On Tue, 26 Jun 2001, Beman Dawes wrote:
bdawes> 
bdawes> But in another message someone claimed that template
bdawes> template parameter replacements were not just broken
bdawes> compiler workarounds, but actually better that template
bdawes> template parameters.  Could someone explain the rationale
bdawes> behind that assertion?

Sure. The template template parameter workaround allows more flexibility
in the template types that are used as parameters. For example, suppose
the template parameter is a container:

#include <vector>

template <template <class U> class C>
struct foo {
  struct bar { };
  C<bar> c;
};

int main()
{
  foo<std::vector> f;
  return 0;
}

Now, what if we don't want to use the default allocator of std::vector,
but some other allocator? How can we pass in std::vector into foo but also
specify some other allocator? With template templates parameters you can
not do this. However, with the workaround it is easy:

#include <vector>

template <class Tag> struct generate_container { };

template <class C_tag>
struct foo {
  struct bar { };
  typename generate_container<C_tag>::template bind<bar>::type c;
};

struct vec_tag { };

template<> struct generate_container<vec_tag> {
  template <class T> struct bind { 
    typedef std::vector<T, std::allocator<T> > type;
  };
};

int main()
{
  foo<vec_tag> f;
  return 0;
}

Cheers,
Jeremy

----------------------------------------------------------------------
 Jeremy Siek                        www: http://www.lsc.nd.edu/~jsiek/
 Ph.D. Candidate, IU B'ton          email: jsiek_at_[hidden]
 Summer Manager, AT&T Research      phone: [phone]
----------------------------------------------------------------------