Templates and overloadering operator << [duplicate]

Answer 1

Your immediate problem is that the operator needs to be templated.

However, your ultimate goal can't be attained through templates alone as dereferencing a IBase* gets you a IBase& - template instantiation occurs at compile time and the compiler has no access to the runtime type.

So your templated operator will never be used if you pass a dereferenced IBase* to operator <<.

Instead, add a virtual output function to the base and override it:

class IBase
{
public:
    IBase() {};
    virtual ~IBase() {};
    virtual std::ostream& output(std::ostream&) const = 0;
};

template 
class Derived
    : public IBase
{
public:
    Derived(T data);
    virtual std::ostream& output(std::ostream& os) const
    {
        os << data;
        return os;
    }
private:
    T data_;
};

std::ostream& operator<<(std::ostream& os, const IBase& dt)
{
    return dt.output(os);
}