Question

Suppose I had the following class:

template<typename T>
class Value {
public:
    Value(std::string name, T value) : name(name), value(value) { }
    T const &value() throw() { return value }
    std::string const &name() throw() { return name }
private:
    std::string name;
    T value;
};

and then I wanted to do something like:

class Group {
public:
    Group(std::string name) : name(name) { }

    template<typename T>
    void add_value(std::string const &name, T const &val) throw() {
        Value<T> tmp(name, val);
        values.insert(tmp);
        return;
    }

private:
    std::string name;
    std::set< Value<????> > values; // HERE BE MY QUESTION
}

Concretely, I want to store Value in its many template forms in the same std::set (like, having a Value<int> and a Value<float> stored in the same set).

Answer 1

You can’t because Value<T> and Value<X> are completely and totally different and nearly unrelated types, like int and string (when T != X). In your case, they are extremely different because they actually store a T in themselves, which means that if the different Ts are different sizes, the Value<T>s will be different sizes. (That’s not the reason, it’s just a reason.)

You may want to try using polymorphism, making Value<T> inherit from a base class so that you can store pointers to that base class which can be pointing to a Value<T> underneath, where T is any type your template supports.