How to let a variable be dependent on other variables inside a class?

Question

What is wrong with the variable international_standard_book_number? How can I make it that it changes, whenever isbn_field_i changes?

#include <iostream>
#include <string>

class ISBN
{
private:
  unsigned int isbn_field_1 = 0;
  unsigned int isbn_field_2 = 0;
  unsigned int isbn_field_3 = 0;
  char digit_or_letter = 'a';
  std::string international_standard_book_number =
    std::to_string(isbn_field_1) + "-" + std::to_string(isbn_field_2) + "-" +
    std::to_string(isbn_field_3) + "-" + digit_or_letter;

public:
  ISBN()
  {
    isbn_field_1 = 0, isbn_field_2 = 0, isbn_field_3 = 0, digit_or_letter = 'a';
  }
  ISBN(unsigned int a, unsigned int b, unsigned int c, char d)
  {
    isbn_field_1 = a, isbn_field_2 = b, isbn_field_3 = c, digit_or_letter = d;
  }
  friend std::ostream& operator<<(std::ostream& os, ISBN const& i)
  {
    return os << i.international_standard_book_number;
  }
};

int
main()
{
  ISBN test(1, 2, 3, 'b');
  std::cout << test << "\n";
  return 0;
}

Output:

0-0-0-a

Desired output:

1-2-3-b

Edit: This question aims at something else(why, instead of how) than mine, and its answers don't help me as much as the answers from this topic.

Answer 1

What is wrong with the variable international_standard_book_number? How can I make it that it changes, whenever isbn_field_i changes?

Generally speaking: you have to reassign it every time one component changes.

In your particular case: change the constructor using initialization list.

I mean... instead

ISBN(unsigned int a, unsigned int b, unsigned int c, char d)
 {isbn_field_1=a, isbn_field_2=b, isbn_field_3=c, digit_or_letter=d;};

write

ISBN(unsigned int a, unsigned int b, unsigned int c, char d)
 : isbn_field_1{a}, isbn_field_2{b}, isbn_field_3{c}, digit_or_letter{d}
{}

Now your example code write

1-2-3-b

What changes ?

With

ISBN(unsigned int a, unsigned int b, unsigned int c, char d)
 {isbn_field_1=a, isbn_field_2=b, isbn_field_3=c, digit_or_letter=d;};

first your fields are default initialized, so

isbn_field_1    = 0;
isbn_field_2    = 0;
isbn_field_3    = 0;
digit_or_letter = 'a';

international_standard_book_number="0"+"-"+"0"+"-"+"0"+"-"+'a';

then is executed the body of the constructor

isbn_field_1    = 1;
isbn_field_2    = 2;
isbn_field_3    = 3;
digit_or_letter = 'b';

but international_standard_book_number remain unchanged.

With

ISBN(unsigned int a, unsigned int b, unsigned int c, char d)
 : isbn_field_1{a}, isbn_field_2{b}, isbn_field_3{c}, digit_or_letter{d}
{}

the initialization list initialize the fields (and substitute the default initialization)

isbn_field_1    = 1;
isbn_field_2    = 2;
isbn_field_3    = 3;
digit_or_letter = 'b';

and then is executed the default initialization of international_standard_book_number but using the new values, so

international_standard_book_number="1"+"-"+"2"+"-"+"3"+"-"+'b';