Question

Consider the following code:

struct Foo
{
    Foo operator+(const Foo &rhs) const;
    // notice lack of: Foo operator*(const Foo &rhs) const;
};

template <class T>
struct Bar
{
    T x, y;
    T add() const { return x + y; }
    T mul() const { return x * y; }
};

I have two questions:

1. Can I inherit from Bar<Foo> and override mul() to something meaningful?

2. Can I inherit from Bar<Foo> without overriding mul() if I never use mul() anywhere?

Answer 1

1. sure
2. sure

Templates are really a kind of smart preprocessor, they’re not compiled. If you don’t use something, you can write complete (syntactically correct) rubbish, i.e you may inherit from

template <class T>
struct Bar
{
    T x, y;
    T add() const { return x + y; }
    T mul() const { return x.who cares what-s in here; }
};

P.S. since your + operator is used in a const function, it should be declared as const too.

EDIT: OK, not all compilers support this, here’s one that compiles with gcc:

template <class T>
struct Bar
{
    T x, y;
    T add() const { return x + y; }
    T mul() const { T::was_brillig & T::he::slith(y.toves).WTF?!0:-0; }
};