c++ - When do programmers use Empty Base Optimization (EBO)

Question

I was reading about Empty Base Optimization(EBO). While reading, the following questions popped up in my mind:

1. What is the point of using Empty class as base class when it contributes nothing to the derived classes (neither functionality-wise, nor data-wise)?

2. In this article, I read this:

//S is empty
class struct T : S
{
??????int x;
};

[...]

Notice that we didn’t lose any data or code accuracy: when you create a standalone object of type S, the object’s size is still 1 (or more) as before; only when S is used as base class of another class does its memory footprint shrink to zero. To realize the impact of this saving, imagine a vector that contains 125,000 objects. The EBO alone saves half a megabyte of memory!

Does it mean that if we don't use "S" as base class of "T", we would necessarily consume double of megabyte of memory? I think, the article compares two different scenarios which I don't think is correct.

I would like to know a real scenario when EBO can proven to be useful.(means, in the same scenario, we would necessarily be at loss IF we don't use EBO!).

Please note that if your answer contains explanations like this :

The whole point is that an empty class has non-zero size, but when derived or deriving it can have zero size, then I'm NOT asking that, as I know that already. My question is, why would anyone derive his class from an empty class in the first place? Even if he doesn't derive and simply writes his class (without any empty base), is he at loss in ANY way?

Answer 1

EBO is important in the context of policy based design, where you generally inherit privately from multiple policy classes. If we take the example of a thread safety policy, one could imagine the pseudo-code :

class MTSafePolicy
{
public:
  void lock() { mutex_.lock(); }
  void unlock() { mutex_.unlock(); }

private:
  Mutex mutex_;
};

class MTUnsafePolicy
{
public:
  void lock() { /* no-op */ }
  void unlock() { /* no-op */ }
};

Given a policy based-design class such as :

template<class ThreadSafetyPolicy>
class Test : ThreadSafetyPolicy
{
  /* ... */
};

Using the class with a MTUnsafePolicy simply add no size overhead the class Test : it's a perfect example of don't pay for what you don't use.