C++ template-function -> passing a template-class as template-argument - c++

Question

i try to make intensive use of templates to wrap a factory class:
The wrapping class (i.e. classA) gets the wrapped class (i.e. classB) via an template-argument to provide 'pluggability'.
Additionally i have to provide an inner-class (innerA) that inherits from the wrapped inner-class (innerB).
The problem is the following error-message of the g++ "gcc version 4.4.3 (Ubuntu 4.4.3-4ubuntu5)":
sebastian#tecuhtli:~/Development/cppExercises/functionTemplate$ g++ -o test test.cpp
test.cpp: In static member function ‘static classA<A>::innerA<iB>* classA<A>::createInnerAs(iB&) [with iB = int, A = classB]’:
test.cpp:39: instantiated from here
test.cpp:32: error: dependent-name ‘classA::innerA<>’ is parsed as a non-type, but instantiation yields a type
test.cpp:32: note: say ‘typename classA::innerA<>’ if a type is meant
As you can see in the definition of method createInnerBs, i intend to pass a non-type argument. So the use of typename is wrong!
The code of test.cpp is below:
class classB{
public:
template < class iB>
class innerB{
iB& ib;
innerB(iB& b)
:ib(b){}
};
template<template <class> class classShell, class iB>
static classShell<iB>* createInnerBs(iB& b){
// this function creates instances of innerB and its subclasses,
// because B holds a certain allocator
return new classShell<iB>(b);
}
};
template<class A>
class classA{
// intention of this class is meant to be a pluggable interface
// using templates for compile-time checking
public:
template <class iB>
class innerA: A::template innerB<iB>{
innerA(iB& b)
:A::template innerB<iB>(b){}
};
template<class iB>
static inline innerA<iB>* createInnerAs(iB& b){
return A::createInnerBs<classA<A>::template innerA<> >(b); // line 32: error occurs here
}
};
typedef classA<classB> usable;
int main (int argc, char* argv[]){
int a = 5;
usable::innerA<int>* myVar = usable::createInnerAs(a);
return 0;
}
Please help me, i have been faced to this problem for several days.
Is it just impossible, what i'm trying to do? Or did i forgot something?
Thanks, Sema

Answer 1

Line 32 should read:
return A::template createInnerBs<innerA>(b);
since createInnerBs is dependent on the template parameter A.
You'll also need to make the constructors of innerA and innerB public.

Answer 2

Here is the corrected code which compiles for me:
class classB{
public:
template < class iB>
class innerB{
iB& ib;
public:
innerB(iB& b)
:ib(b){}
};
template<template <class> class classShell, class iB>
static classShell<iB>* createInnerBs(iB& b){
// this function creates instances of innerB and its subclasses,
// because B holds a certain allocator
return new classShell<iB>(b);
}
};
template<class A>
class classA{
// intention of this class is meant to be a pluggable interface
// using templates for compile-time checking
public:
template <class iB>
class innerA: public A::template innerB<iB>{
public:
innerA(iB& b)
: A::template innerB<iB>(b){}
};
template<class iB>
static inline innerA<iB>* createInnerAs(iB& b);
};
template<class A>
template<class iB>
inline classA<A>::innerA<iB>* classA<A>::createInnerAs(iB& b)
{
return A::template createInnerBs<classA::template innerA>(b);
}
typedef classA<classB> usable;
int main (int argc, char* argv[]){
int a = 5;
usable::innerA<int>* myVar = usable::createInnerAs(a);
return 0;
}
Even if I think you're overcomplicating things... But I don't fully understand your use case.