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

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.

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';