C++

15.5    Derived Types Extend

In Chapter 12, Listing 12.24, Easter was a free function. Wouldn't it make sense if Easter were a method of Year? Then we could directly ask an instance Year year{2018} with year.easter() which day Easter falls on in that year.

// https://godbolt.org/z/zqKvMedz7 (full example)class Date;                         // forward declarationclass Year : public Value {public:    explicit Year(int v) : Value{v, 4} {}    Date easter() const;            // declare new method};// Declare Month, Day, and Date here. Then:Date Year::easter() const {         // define new method    const int y = value_;    int a = value_/100*1483 - value_/400*2225 + 2613;    int b = (value_%19*3510 + a/25*319)/330%29;    b = 148 - b - (value_*5/4 + a - b)%7;    return Date{Year{value_}, Month{b/31}, Day{b%31 ...