Const Class Objects and Const Member Functions

Const Class Objects and Const Member Functions

Declare an object as const and it becomes read-only: it cannot be changed after you build it. To match that promise, const objects may only call const member functions - methods that promise not to modify the object.

A const object

const Car myCar;        // read-only object
cout << myCar.getYear();   // OK only if getYear() is const

Trying to call a non-const method on a const object fails at compile time.

A const member function

Write const after the parameter list:

class Car {
  int year;
public:
  int getYear() const {    // const function
    return year;
  }
  void setYear(int y) { year = y; }   // not const
};

• The const guarantees the method will not modify the object.
• A const method can read members but cannot write them.

Why the rule exists

• Compiler catches mistakes at build time instead of runtime.
• const documents intent: the call cannot change anything.
• Const references can be passed safely everywhere without fear of mutation.

TL;DR

• A const object is read-only after construction.
• Const objects may only call const methods.
• Mark a function const with void print() const;.
• A const method promises it will never modify the object.