David Greene wrote:

Well, my mpl adventure continues.

I'm trying to place pair members of a sequence with their keys:

#include #include #include #include #include #include #include

#include <iostream> #include <typeinfo>

using namespace boost; using namespace boost::mpl;

struct pair_base {};

template struct pair_type : public pair, private pair_base {};

template<typename Sequence> struct replaceit { typedef typename replace_if< Sequence, is_base_and_derived, key_type, _> >::type type; };

int main(void) { replaceit< vector< charm

You should be seeing a syntax error right here.

pair_type >, pair_type > double >

...

::type toprint;

std::cout << typeid(toprint).name() << std::endl;

return(0); };

I know this is incorrect because the pairs will be replaced by the key_type metafunction itself.

If I add ::type to the end of key_type, _> the compiler complains that no ::first member exists:

What's the right way to do this?

Use an algorithm like transform that _computes_ the new values rather than simply replacing them with a constant value: template <class Seq> struct replaceit : transform< Seq , if_< is_base_and_derived , key_type,_> , _ > > {};

Furthermore, is there a better way to identify pairs in replace_if than deriving from a special class? All I can think of is writing a predicate class and specializing it for mpl::pair. Seems like there should be an easier way.

Why? That seems like a pretty darned good approach to me. -- Dave Abrahams Boost Consulting http://www.boost-consulting.com