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.