David Abrahams wrote:
I suppose not. What I really wanted was the ability to take a
regular old template class and create a generator out of it:
template<typename T, typename U, typename V>
struct my_type { ... } // Note: no ::type member
typedef SHAZAM<my_type<int, _1, _2> > generator;
typedef generator::template apply<char, float>::type my_type_inst;
I want SHAZAM to be generic enough to take any template class,
not just my_type.
You can do that, up to N arguments, for some N.
But consider this testcase:
//#define CT_DEBUG
//#include "ct_print.hh"
#include <boost/mpl/placeholder.hpp>
#include <boost/mpl/lambda.hpp>
template<typename T, typename U, typename V>
struct my_type {
};
template<typename T, typename U, typename V>
struct my_type_generator {
typedef my_type<T, U, V> type;
};
template<template<typename, typename, typename> class Base,
typename T, typename U, typename V>
struct trivial_generator {
typedef Base<T, U, V> type;
};
int main(void)
{
typedef my_type_generator<char, short, int> generator;
//CT_PRINT(my_type_generator_result_is, generator::type);
typedef trivial_generator<my_type, char, short, int> t_generator;
//CT_PRINT(trivial_generator_result_is, t_generator::type);
// Try to make some generators
typedef boost::mpl::lambda<my_type_generator<char, boost::mpl::_1,
boost::mpl::_2> >::type lambda_generator;
typedef lambda_generator::apply<short, int> lg_apply_result;
//CT_PRINT(lambda_generator_result_is, lg_apply_result::type);
// Does not work
typedef boost::mpl::lambda<trivial_generator<my_type, char,
boost::mpl::_1, boost::mpl::_2> >::type lambda_trivial_generator;
// g++ 3.2 claims apply doesn't exist.
typedef lambda_trivial_generator::apply<short, int> ltg_result;
//CT_PRINT(lambda_trivial_generator_result_is, ltg_result::type);
return(0);
}
Here's what g++ 3.2 tells me:
lambda_test.cc: In function `int main()':
lambda_test.cc:38: ISO C++ forbids declaration of `apply' with no type
lambda_test.cc:38: template-id `apply<short int, int>' used as a
declarator
lambda_test.cc:38: typedef name may not be class-qualified
lambda_test.cc:38: parse error before `;' token
But I can construct a generic trivial generator:
template<template<typename A, typename B, typename C> class Base,
typename T, typename U, typename V>
struct trivial_generator {
typedef Base<T, U, V> type;
};
Then I can use MPL lambda facilities. Unfortunately, I need a
trivial_generator for every arity of class template.
Only as part of the implementation. You just need a nice wrapper over
the top to hide it all.
Right. Can you point me to the MPL equivalent as a guide of
how to do this?
-Dave
--
"Some little people have music in them, but Fats, he was all music,
and you know how big he was." -- James P. Johnson
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