C++ FAQ / Section 35 / FAQ 35.22

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Revised Jul 4, 2012

[35.22] Follow-up to previous: can I pass in the underlying structure and the element-type separately?

Yes, with a "proxy" trick.

Here's the problem: std::vector template can have, does have, more than one argument. You're required to make them match in the number, order, and nature — type/non-type, etc.

It is possible, however, to "cheat" your way out of specifying all those arguments and use the defaults. It's called the "proxy template" technique:

#include <vector>
#include <list>

template<typename T>
struct wrap_vector {
  typedef std::vector<T> type;
};

template<typename T>
struct wrap_list {
  typedef std::list<T> type;
};

template<typename T, template<typename> class C>
struct A {
  typename C<T>::type data;  // trick to use a proxy
};

int main()
{
  A<int,wrap_vector> avi;
  A<double,wrap_list> adl;
  ...
}

You can also create a proxy if you actually have a template that takes only one argument:

template<typename T>
struct wrap_my1argtemplate {
  typedef my1argtemplate<T> type;
};

The "template typedefs" proposal will allow redefining templates just like we do with types, which will make the proxy-trick unnecessary. Until then, use something like the above.