C++ class with template member variable

Question

I am trying to solve a programming problem that consists of an object (call it Diagram), that contains several parameters. Each parameter (the Parameter class) can be one of

Answer 1

You got very close. I added a few bits because they're handy

class ParameterBase
{
public:
    virtual ~ParameterBase() {}
    template const T& get() const; //to be implimented after Parameter
    template void setValue(const U& rhs); //to be implimented after Parameter
};

template 
class Parameter : public ParameterBase
{
public:
    Parameter(const T& rhs) :value(rhs) {}
    const T& get() const {return value;}
    void setValue(const T& rhs) {value=rhs;}    
private:
    T value;
};

//Here's the trick: dynamic_cast rather than virtual
template const T& ParameterBase::get() const
{ return dynamic_cast&>(*this).get(); }
template void ParameterBase::setValue(const U& rhs)
{ return dynamic_cast&>(*this).setValue(rhs); }

class Diagram
{
public:
    std::vector v;
    int type;
};

Diagram can then do stuff like these:

Parameter p1("Hello");
v.push_back(&p1);
std::cout << v[0]->get(); //read the string
v[0]->set("BANANA"); //set the string to something else
v[0]->get(); //throws a std::bad_cast exception

It looks like your intent is to store resource-owning pointers in the vector. If so, be careful to make Diagram have the correct destructor, and make it non-copy-constructable, and non-copy-assignable.