Initializing template base-class member types in derived-class initializer lists

Here is some code outlining a problem I've been wrestling with. The final problem (as far as g++ is concerned at the moment) is that: "error: 'Foo-T' was not declared in this scope" when performing the Bar::Bar(...) constructor routine. Otherwise, the problem I'm attempting to learn my way through is one of setting base-class member types based on arguments passed to a derived-class constructor using templates. If there were a way to set the base-class member type (T Foo-T) simply by passing arguments to the derived-class constructor, I would prefer that. As of now I can't see a way past using both the template argument and a matching derived-class constructor argument to accomplish this task. Can you spot anything in the following code that I can be doing better to achieve the same goals? I'm rather new to generic-coding and templates.

#include <iostream>
typedef int a_arg_t;
typedef double b_arg_t;
typedef std::string foo_arg_t;

class TypeA {
public:
    TypeA ();
    TypeA (a_arg_t a) {
      /* Do sosmething with the parameter passed in */
    }

};

class TypeB {
public:
    TypeB ();
    TypeB (b_arg_t b) {
      /* typeB's constructor - do something here */
    }

};

// The base-class with a member-type to be determined by the template argument
template <class T>
class Foo {

public:
    Foo (const foo_arg_t foo_arg) : _foo_arg(foo_arg) // initialize something here
    {
      /* do something for foo */
    }
    T Foo_T; // either a TypeA or a TypeB - TBD
    foo_arg_t _foo_arg;
};

// the derived class that should set the basse-member type (T Foo_T)
template <class T>
class Bar : public Foo<T> {
public:
    Bar (const foo_arg_t bar_arg, const a_arg_t a_arg)
    : Foo<T>(bar_arg) // base-class initializer
    {
        // the initialization of Foo_T has to be done outside the initializer list because it's not in scsope until here
        Foo_T = TypeA(a_arg); // if an a_arg_t is passed in, then we set the Fo