20th April 2023, 1 min read

Defining interfaces in C++: concepts versus inheritance

In a previous blog post, I showed how you could define ‘an interface’ in C++ using concepts. For example, I can specify that a type should have the methods has_next, next and reset:

template <typename T>
concept is_iterable = requires(T v) {
                        { v.has_next() } -> std::convertible_to<bool>;
                        { v.next() } -> std::same_as<uint32_t>;
                        { v.reset() };
                      };

I can then define a function template, taking a concept instance as a parameter:

template <is_iterable T> size_t count(T &t) {
  t.reset();
  size_t count = 0;
  while (t.has_next()) {
    t.next();
    count++;
  }
  return count;
}

In that blog post, I stated that I did not take into account inheritance as a strategy. Let us do so. We can define a generic base class and a corresponding generic function:

class iter_base {
public:
  virtual bool has_next() = 0;
  virtual uint32_t next() = 0;
  virtual void reset() = 0;
  virtual ~iter_base() = default;
};

size_t count_inheritance(iter_base &t) {
  t.reset();
  size_t count = 0;
  while (t.has_next()) {
    t.next();
    count++;
  }
  return count;
}

I can define a class that is suitable for both functions, as it satisfies the inheritance condition, as well as the concept:

struct iterable_array : iter_base {
  std::vector<uint32_t> array{};
  size_t index = 0;
  void reset() { index = 0; }
  bool has_next() { return index < array.size(); }
  uint32_t next() {
    index++;
    return array[index - 1];
  }
};

So far so good. But what is the difference between these two expressions given that a is an instance of iterable_array?

• count(a),
• count_inheritance(a).

Given an optimizing compiler, the first function (count(a)) is likely to just immediately return the size of the backing vector. The function is nearly free.

The second function (count_inheritance(a)) does not know anything about the iterable_array type so it will iterate through the content naively, and might be hundreds of times more expensive.