Re: simple template question

Hi everyone,

I'm trying to create two simple template classes where one of them
depends on the other to initialize its member integer variable. After
some time, I decided to post a message here because I don't really
understand why I can't make this work.

template<int n>
class ClassA
{
public:
int n_;

ClassA() : n_(n)
{}
};

template<class ClassA>
class ClassB
{
public:
int m_;

ClassB() : m_(ClassA::n_)
{}
};

int main()
{
typedef ClassA<2> ca;
typedef ClassB<ca> cb;

cb cb1;
return 1;
}

The compiler errors that I get:

aaragon@aaragon-laptop:~/Desktop$ g++ test.cxx
test.cxx: In constructor 'ClassB<ClassA>::ClassB() [with ClassA =
ClassA<2>]':
test.cxx:28: instantiated from here
test.cxx:5: error: object missing in reference to 'ClassA<2>::n_'
test.cxx:17: error: from this location

I guess that the error lies on the fact that a typedef is not an
instantiation of the object so therefore I cannot call the variable n
within ClassA.

On the other hand, I cannot instantiate ClassA to create the type
definition of classB because I receive a compiler error that tells me that
the instantiated object of ClassA cannot appear in a constant expression.

Is there a way around this? Maybe the solution is real simple. Thank
you.