Step 8 of 9

Keeping invariants

An invariant is a rule that must be true for every object, at all times, from the outside. For a health bar: 0 <= hp <= max.

This is the real reason for private: if only member functions can touch the data, only they can break the rule, and you check it in one place. Every constructor must establish the invariant and every member function must preserve it.

void damage(int amount) {
    hp_ = std::max(0, hp_ - amount);   // never below 0
}

Your turn: write class Health:

• Health(int max): max below 1 becomes 1; starts full
• void damage(int n) and void heal(int n): negative amounts are ignored; hp stays within 0..max
• int hp() const, int max() const, bool alive() const

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