Make Interfaces Hard to Misuse

Don't push the responsibility of maintaining invariants on the caller.

Bad

class Vector {
  explicit Vector(int num_slots);
  int RemainingSlots() const;
  void AddSlots(int num_slots);
  void Insert(int value);
};

In the code above, the caller needs to check RemainingSlots, and if 0, AddSlots in order for Insert to work properly.

Instead, Insert could automatically manage slots.

Good

class Vector {
  explicit Vector(int num_slots);
  void Insert(int value);
};

Other signs an interface is hard to use:

• Initialize / Deitialize functions.
• Allowing partially created objects.
• Parameters that can have invalid values.

Sometimes it's not practical to have a foolproof interface. In those cases rely on static analysis or documentation.