Const Method

A member function with the const keyword after its parameter list cannot modify its object's fields:

class Counter {
public:
  int get() const { return count; }
  void reset() const { count = 0; }  // ERROR.

private:
  int count;
};

A const object can only call its const methods:

class Counter {
public:
  Counter() : count(0) {}
  int get() const { return count; }
  void increment() { count++; }

private:
  int count;
};

const Counter c;
c.get();        // OK.
c.increment();  // ERROR.

This also applies when an object is passed by const reference:

void print(const Counter& c) {
  std::cout << c.get() << "\n";  // OK.
  c.increment();                 // ERROR.
}

When a const method is defined outside the class, const must appear on both the declaration and the definition:

class Counter {
public:
  int get() const;

private:
  int count;
};

int Counter::get() const {
  return count;
}