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 several types: int, double, complex, string - to name a few.

So my first instinct was to define my Diagram class as having a vector of template parameters, which would look like this.

class Diagram { private:     std::vector<Parameter<T> > v; }; 

This doesn't compile, and I understand why. So, based on the recommendations on this page How to declare data members that are objects of any type in a class, I modified my code to look like:

class ParameterBase { public:     virtual void setValue() = 0;     virtual ~ParameterBase() { } };   template <typename T> class Parameter : public ParameterBase { public:     void setValue() // I want this to be                      // void setValue(const T & val)     {         // I want this to be          // value = val;     }  private:     T value; };  class Diagram { public:     std::vector<ParameterBase *> v;     int type; }; 

I'm having trouble figuring out how to call the setValue function with an appropriate templated parameter. It is not possible to have a templated parameter in the ParameterBase abstract base class. Any help is greatly appreciated.

P.S. I don't have the flexibility to use boost::any.

Answer 1

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

class ParameterBase { public:     virtual ~ParameterBase() {}     template<class T> const T& get() const; //to be implimented after Parameter     template<class T, class U> void setValue(const U& rhs); //to be implimented after Parameter };  template <typename T> 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<class T> const T& ParameterBase::get() const { return dynamic_cast<const Parameter<T>&>(*this).get(); } template<class T, class U> void ParameterBase::setValue(const U& rhs) { return dynamic_cast<Parameter<T>&>(*this).setValue(rhs); }  class Diagram { public:     std::vector<ParameterBase*> v;     int type; }; 

Diagram can then do stuff like these:

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