Elegant way of making template arguments dependent on user input?

Hey everybody,

I got into serious trouble with template programming. I have a class
which uses three template arguments, say

template<typename Atype, typename Btype, typename Ctype>
class some_class {
};

and I want to make the values of these template arguments dependent on
the input of the program as shown by the following pseudocode:

if (input_for_Atype == "a")
   firsttype := first_A;
else if (input_for_Atype == "b")
   firsttype := second_A;
//...

if (input_for_Btype == "1")
   secondtype := first_B;
else if (input_for_Btype == "2")
   secondtype := second_B;
//...

// analogous for Ctype

// instanciate correct version of "some_class"
some_class<firsttype, secondtype, thirdtype> instance_of_class;

// Begin doing stuff with the class
instance_of_class.do_something();
//...
// End

I hope you get the idea.

The number of choices for each template argument is limited of course,
so a canonical solution would be to write out each possible combination
of the template arguments. This is really ugly, since in my case there
are already 24 possible combinations (2*4*3) for the template arguments,
and there will most likely be more in the future. Also the code would be
redundant, as the part between "Begin doing stuff" and "end" is always
the same for each of the combinations.

My question is whether there is a more elegant way to solve my problem.

I have already tried to do the following. For each of the template
arguments write a function which decides (dependant on the input) the
type of its related template, like this:

template<typename Atype, typename Btype, typename Ctype>
inline void run() {
    some_class<Atype, BType, Ctype> instance_of_class();
    // do the stuff... code is only written once!
}

template<typename Atype, typename Btype>
inline void choose_C() {
    if (input_for_Ctype == 1)
       run<Atype, Btype, first_C>();
    else if (input_for_Ctype == 2)
       run<Atype, Btype, second_C>();
    // ...
}

template<typename Atype>
inline void choose_B() {
    if (input_for_Btype == 1)
       choose_C<Atype, first_B>();
    else if (input_for_Btype == 2)
       choose_C<Atype, second_B>();
    //...
}

inline void choose_A() {
    if (input_for_Atype == 1)
       choose_B<first_A>();
    else
       choose_B<second_A>();
}

int main() {
    choose_A();
}

Unfortunately this won't work in my case, since the value of the Atype
template also depends on "input_for_Ctype". In fact, the method choose_A
looks like this:

inline void choose_A() {
    if (input_for_Atype == 1 || input_for_Ctype == 1)
       choose_B<first_A>();
    else
       choose_B<second_A>();
}

Now, it is crucial for input_for_Ctype == 1 to use first_A as the Atype,
since it defines some member variables which aren't available in
second_A. Unfortunately my C++ compiler seems to compile the code in the
run method for Atype = second_A and Ctype = first_C as well, though this
path is never taken in the program. This is leading to errors that
second_A won't have the requested members. Declaring all functions
inline as shown above didn't help.

I hope this wasn't too much at once, but the situation seems to be
rather complex. If there are any questions to the problem just feel free
to ask them and I will try to explain it in more detail.

Is there any solution for this or is it just impossible in C++? Of
course an alternative would be to do it with class inheritance and
virtual methods, but this is no option, since performance is most
important (I'm testing algorithms on huge datasets, taking hours of time
already). "Googling" on this problem didn't really reveal anything of use.

Greets and thanks in advance,
Thomas

PS: Maybe this is of use:

# gcc -v
Using built-in specs.
Target: x86_64-suse-linux
Configured with: ../configure --enable-threads=posix --prefix=/usr
--with-local-prefix=/usr/local --infodir=/usr/share/info
--mandir=/usr/share/man --libdir=/usr/lib64 --libexecdir=/usr/lib64
--enable-languages=c,c++,objc,fortran,java,ada --enable-checking=release
--with-gxx-include-dir=/usr/include/c++/4.1.0 --enable-ssp
--disable-libssp --enable-java-awt=gtk --enable-gtk-cairo
--disable-libjava-multilib --with-slibdir=/lib64 --with-system-zlib
--enable-shared --enable-__cxa_atexit --enable-libstdcxx-allocator=new
--without-system-libunwind --with-cpu=generic --host=x86_64-suse-linux
Thread model: posix
gcc version 4.1.0 (SUSE Linux)

# uname -a
Linux compute6 2.6.16.13-4-smp #1 SMP Wed May 3 04:53:23 UTC 2006 x86_64
x86_64 x86_64 GNU/Linux