Problem with templates and typedefs

[sigfriedmcwild]

I'm trying to write a class with a member fuction that returns an object of a type that's defined in one of the parameters.
Something like this:

class J
{
        public:
                template<typename T>
                typename T::name op(const T& o)
                {
                        return T::name();
                }
};

T in this case would be an object of this class:

template<typename T>
class TI
{
        public:
                typedef T name;
};

with any random class used as the template parameter of TI
I'm using this:

class I
{
};

when I try to use it like this

#include "ideas2.h" // that's where the class definitions are

int main()
{
        /* other stuff I'm trying (currently commented out) removed */

        J j1;

        TI<I> ti1;

        j1.op(ti1);

        return 0;
}

gcc gives the following errors:

ideas2.h: In member function 'typename T::name J::op(const T&) [with T = TI<I>]':
test.cpp:36:   instantiated from here
ideas2.h:20: error: dependent-name 'T::name' is parsed as a non-type, but instantiation yields a type
ideas2.h:20: note: say 'typename T::name' if a type is meant

gcc says to use typename, but I'm already using it and if I remove it it says this instead:

ideas2.h:18: error: expected constructor, destructor, or type conversion before 'op'
test.cpp: In function 'int main()':
test.cpp:36: error: 'class J' has no member named 'op'

Any ideas?

[Bench82]

You can't assume anything about a generic template parameter, so T::name is illegal where T is a generic typename.

"name" is inside class TI, so use

typename TI<T>::name

[EDIT]also, looking at this line...

return T::name();

'name' is a typename.. not an object... nor a function.. I'm not entirely clear which object you're trying to return from this function. Are you attempting to create a new object of type 'name' ?


One suggestion commonly given to people trying to create templated classes from scratch is to create your class structure(s) first, without templates (use some built-in type like a char or int for testing) ... then once the bugs are ironed out, and it works correctly for that single type, put the templates in.. because you will undoubtedly have additional problems involving templates.

[sigfriedmcwild]

Well...

this compiles fine

class J
{
        public:
                template<typename T>
                void op(const T& o)
                {
                        typename T::name var;
                }
};

in fact

class J
{
        public:
                template<typename T>
                typename T::name op(const T& o)
                {
                        typename T::name var;
                        return var;
                }
};

works fine.

By the way, I'm stupid, stupid stupid.
I kept on looking for errors in the function signature when the error was in the original return statement

[sigfriedmcwild]

[EDIT]also, looking at this line...

return T::name();

'name' is a typename.. not an object... nor a function.. I'm not entirely clear which object you're trying to return from this function. Are you attempting to create a new object of type 'name' ?

Yeah that's what I'm trying to do return a new object of type T::name
Since normally you can do

...
return some_type();

I thought it would work as well. It seems there's some issues when that gets mixed with the typename keyword

This seems to work though

typename T::name p = new typename T::name();

Oh well, thanks forthe help