Question

So, I’m learning C++, and I’ve run into something which I know how to do in Java, but not in C++ :).

I have a template for a container object, which is defined as such:

template <class T>
class Container {
    vector<T> contained;

    public:

    void add(T givenObject) {
        this->contained.push_back(givenObject);
    }

    T get(string givenIdentifier) throw (exception) {
        for (int i = 0; i < this->contained.size(); i++) {
            if (this->contained[i].getIdentifier() == givenIdentifier) {
                return this->contained[i];
            }
        }
        throw new exception("An error has occured which has caused the object you requested to not be found. Please report this bug.");
    }

    bool empty() {
        return this->contained.empty();
    }

    bool identifierExists(string givenIdentifier) {
        for (int i = 0; i < this->contained.size(); i++) {
            if (this->contained[i].getIdentifier() == givenIdentifier) {
                return true;
            }
        }
        return false;
    }
};

This actually works very well, with one small issue. It comes down to two lines: the first is the template definition and the second is

this->contained[i].getIdentifer()

In Java, when declaring a Generic (template) one can define a superclass/interface which all members of T must extend in order to not create an error. However, I’m not sure of a way to do this in C++, and my concern is that coupling the implementation here to a getIdentifier method which might not be defined is bad design.

Now, it’s not a huge deal if that’s the case, this is just a little challenge project to help me learn the language, but I like to try to do things right. Is there a way to do what I’m thinking? I know you can do it with primitives, for instance:

template <int T>

is valid, but when I try to use a user defined class, I get a compiler error. Any suggestions?

Answer 1

It’s not possible for you to put artificial limitations on template type parameters. If the type given doesn’t support the way you use it, you’ll receive a compiler error. A feature called ‘concepts,’ which would essentially allow this, was going to be added to the next C++ standard, but it was delayed to the next-next standard due to time constraints. If T doesn’t have a visible getIdentifier() function, the instantiation won’t compile.

Template parameters need to be deduced at compile time. template<int T> is valid because the first template parameter is an integer; you can instantiate it with any constant integer. If you attempted to use it with a non-const integer variable, it wouldn’t compile. An instance of a class isn’t a compile time constant, so it can’t be used.