Extracting template types from a typedef'd template declaration

[Adam Nielsen]

Hi everyone,

Yet another syntax problem that's baffling me with templates. I want to
instantiate a template with a single parameter as per normal, however
the parameter is actually a template class itself, with all *its*
parameters filled out (in the form of a typedef.) I can't work out how
to break apart the typedef to reveal what data types were used to create
it in the first place.

Here is some example code that demonstrates the problem. I've tried
every syntax I can think of but I haven't managed to hit upon the right
one yet! Any suggestions would be much appreciated.

Many thanks as always,
Adam.


#include <iostream>

template <class TFrom, class TTo>
class CLinkData;

class A { };
class B { };

typedef CLinkData<A, B> MyLink;

// How do you declare the template to accept a CLinkData parameter, but
// broken out into its component types?
//template <template <class TLinkFrom, class TLinkTo> class TLinkData>
//template <template <class TLinkFrom, class TLinkTo>
// class TLinkData<TLinkFrom, TLinkTo> >
template <class CLinkData<TLinkFrom, TLinkTo> >
void createStoredLink(void)
{
std::cerr << "CLinkData types are " << typeid(TLinkFrom).name() <<
" and " << typeid(TLinkTo).name() << std::endl;
// TLinkFrom should be A, and TLinkTo should be B, the types used
// to declare MyLink.
return;
}

int main(void)
{
createStoredLink<MyLink>();
return 0;
}

 

[xtrigger303]

Hi everyone,

Yet another syntax problem that's baffling me with templates. I want to
instantiate a template with a single parameter as per normal, however
the parameter is actually a template class itself, with all *its*
parameters filled out (in the form of a typedef.) I can't work out how
to break apart the typedef to reveal what data types were used to create
it in the first place.

Here is some example code that demonstrates the problem. I've tried
every syntax I can think of but I haven't managed to hit upon the right
one yet! Any suggestions would be much appreciated.

Many thanks as always,
Adam.

#include <iostream>

template <class TFrom, class TTo>
class CLinkData;

class A { };
class B { };

typedef CLinkData<A, B> MyLink;

// How do you declare the template to accept a CLinkData parameter, but
// broken out into its component types?
//template <template <class TLinkFrom, class TLinkTo> class TLinkData>
//template <template <class TLinkFrom, class TLinkTo>
// class TLinkData<TLinkFrom, TLinkTo> >
template <class CLinkData<TLinkFrom, TLinkTo> >
void createStoredLink(void)
{
std::cerr << "CLinkData types are " << typeid(TLinkFrom).name() <<
" and " << typeid(TLinkTo).name() << std::endl;
// TLinkFrom should be A, and TLinkTo should be B, the types used
// to declare MyLink.
return;

}

int main(void)
{
createStoredLink<MyLink>();
return 0;



}- Hide quoted text -

- Show quoted text -

Hi,
try one of the following.
Bye,
Francesco B.

#include <iostream>
#include <typeinfo>

template< typename T >
struct CTypeFromType
{};

template <class TFrom, class TTo>
class CLinkData;


class A { };
class B { };


typedef CLinkData<A, B> MyLink;

// first method

template< typename TFrom, typename TTo >
void createStoredLinkAux( CTypeFromType< CLinkData< TFrom, TTo > > )
{
std::cerr << "CLinkData types are " << typeid(TFrom).name() <<
" and " << typeid(TTo).name() << std::endl;

return;



}

template < typename T >
inline void createStoredLink(void)
{
createStoredLinkAux( CTypeFromType< T >() );
}

// second method

template< typename T >
struct CCreateStoredLink
{};

template< typename TFrom, typename TTo >
struct CCreateStoredLink< CLinkData< TFrom, TTo > >
{
static void Do()
{
std::cerr << "CLinkData types are " << typeid(TFrom).name() <<
" and " << typeid(TTo).name() << std::endl;

return;
}
};

int main(void)
{
createStoredLink<MyLink>();
CCreateStoredLink<MyLink>:o();
//createStoredLink< int >(); //compile time error
//CCreateStoredLink< int>:o(); // compile tiem error
std::cin.get();
return 0;
}

 

[Adam Nielsen]

Hi,

try one of the following.
Bye,
Francesco B.

// first method

I like this one the best, I'll use that. Thanks for your help! That
CTypeFromType idea is turning out to be quite versatile

Cheers,
Adam.

 

[xtrigger303]

I like this one the best, I'll use that. Thanks for your help! That
CTypeFromType idea is turning out to be quite versatile

Cheers,
Adam.

Have a look at Alexandrescu's "Modern C++ Design" Techniques chapter.
There are quite a few interesting things in there.

Most probably there are many other books like that, but I don't have
much time to read...
;-)
Bye,
FB