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

问题

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.

回答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';

回答2:

Use a member function.

#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() const {
        return 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;
}

Variables in c++ use value sematics. When you do

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;

it will assign a value to international_standard_book_number based on the values that isbn_field_n has right now. It does not create some kind of automatic link between these variables that make sure they stay in sync.

If you want that behaviour you would have to make sure you update international_standard_book_number everytime one the the other fields are updated.

回答3:

If you only need to set the value once (e.g. the other values don't change after the object got constructed) you can use an initializer list:

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),
      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)
{};

But keep in mind, that the member are still initialized in the order they are declared, not in the order of the initializer list.

Technically, you don't need to initialize international_standard_book_number in the initializer list, as max66's answer shows, it's a question of personal preference.

回答4:

Maintain class invariants (depends vars) it is what you have to code manually. It is one of reasons why we need classes. In a class you may forbid direct changes of members (make them private), but when they are changed via for instance special set methods update invariants accordingly.

E.g.

void set_field_1(int field) {
  isbn_field_1 = field;
  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; 
}

回答5:

I want to add to @max66's answer.

Usually you have a hierarchy of constructors calling each other and one final final "master" constructor that takes all arguments and initializes the variables. This avoids code duplication and vastly simplifies which constructors initialize what. You can see what I mean in the below example. Besides that, to do proper string formatting in a readable manner, use the {fmt} library:

#include <fmt/format.h>
#include <string>

class ISBN
{
private:
        unsigned int isbn_field_1;
        unsigned int isbn_field_2;
        unsigned int isbn_field_3;
        char digit_or_letter;
        std::string international_standard_book_number;

public:
        ISBN()
                : ISBN{ 0, 0, 0, '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 }
                , international_standard_book_number{ fmt::format("{}-{}-{}-{}",
                                                                  isbn_field_1,
                                                                  isbn_field_2,
                                                                  isbn_field_3,
                                                                  digit_or_letter) }
        {}
};

回答6:

This code in Visual Studio 2019 at least works:

#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(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), 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)
    {
    }
    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";
    test = {2, 3, 4, 'c'};
    std::cout << test << "\n";
    return 0;
}

Also, why the empty constructor?

来源：https://stackoverflow.com/questions/60652051/how-to-let-a-variable-be-dependent-on-other-variables-inside-a-class