struct usage and setup
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Thread: struct usage and setup

  1. #1
    Join Date
    Apr 2013
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    struct usage and setup

    Hi,
    I have included a small program that revolves around struct's. This program works to a certain extent. I have three questions about the struct form and how it is used:
    1.) Is there a way to enter data without asking the user how many records you want to add (howmany)?
    2.) Is there a way to to make the struct persistent. In other words can you end the program and then start the program again and then add more data to the struct?
    3.) What are some of the uses of the struct?
    Thanks

    Code:
    //
    #include <iostream>
    
    const int strsize = 30;
    
    // Benevolent Order of Programmers name structure
    struct bop {
        char fullname[strsize]; //real name
        char title[strsize];    //job title
        char bopname[strsize];  //secret bop name
        int preference;         //0 = fullname, 1 = title, 2 = bopname
    };
    
    int main()
    {
        using namespace std;
        int howmany = 0;
        cout << "How many bop's are you going to load into the bop structure?: ";
        cin >> howmany;
        cin.get();
        int count = 0;
        bop * ps = new bop[howmany];
    
        while (count < howmany)
        {
          cout << "\n" << "Please enter bop title: ";
          cin.get(ps[count].title, strsize);
          cin.get();
          cout << "\n" <<"Please enter bop fullname: ";
          cin.get(ps[count].fullname, strsize);
          cin.get();
          cout << "\n" <<"Please enter bopname: ";
          cin.get(ps[count].bopname, strsize);
          cin.get();
          cout << "\n" << "Please enter preference: ";
          cin >> ps[count].preference;
          cin.get();
          /*
          cout << "bop title: " << ps[count].title << "\n";
          cout << "bop fullname: " << ps[count].fullname << "\n";
          cout << "bopname: " << ps[count].bopname << "\n";
          cout << "preference: " << ps[count].preference << "\n";
          */
          count++;
        }
        cout << "Please enter one of the following choices:\n";
        cout << "a. display by name\t" << "b. display by title\n";
        cout << "c. display by bopname\t" << "d. display by preference\n";
        cout << "q. quit\n";
        cout << "Enter  choice ";
        char choice;
        cin >> choice;
        while (choice != 'Q' && choice != 'q')
        {
            switch(choice)
            {
                case 'a':
                case 'A':
                        count = 0;
                        while (count < howmany)
                           {
                           cout << ps[count].fullname << "\n";
                           count++;
                           }
                          break;
                case 'b':
                case 'B':
                        count = 0;
                        while (count < howmany)
                           {
                           cout << ps[count].title << "\n";
                           count++;
                           }
                           break;
                case 'c':
                case 'C':
                        count = 0;
                        while (count < howmany)
                           {
                           cout << ps[count].bopname << "\n";
                           count++;
                           }
                           break;
                case 'd':
                case 'D': cout << "games are for the young at heart \n";
                          break;
                default : cout << "Enter a valid choice.\n";
             }
             cout << "Next choice: ";
             cin >> choice;
         }
    
    
        cin.get();
        cin.get();
        return 0;
    }

  2. #2
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    Re: struct usage and setup

    1) Yes. Use a vector and use STL class string instead of char[]. If you use vector with your current struct then you will need to implement constructor/destructor/copy constructor and operator= functions.

    2) Yes. Read/Write the records to a file. Read from file at the start of the program and write the records to the file at the end.

    preference would be better as an enum as it takes only 3 values.
    All advice is offered in good faith only. You are ultimately responsible for effects of your programs and the integrity of the machines they run on.

  3. #3
    GCDEF is offline Elite Member Power Poster
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    Re: struct usage and setup

    You didn't ask for one, but I'll give you an English lesson anyway. Apostrophes are used to indicate possession and contraction. They're not used to pluralize words.

  4. #4
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    Re: struct usage and setup

    Quote Originally Posted by 2kaud View Post
    1) Yes. Use a vector and use STL class string instead of char[]. If you use vector with your current struct then you will need to implement constructor/destructor/copy constructor and operator= functions.

    2) Yes. Read/Write the records to a file. Read from file at the start of the program and write the records to the file at the end.

    preference would be better as an enum as it takes only 3 values.
    Thanks a lot, I'll look into it,
    Bob

  5. #5
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    Re: struct usage and setup

    Thanks

  6. #6
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    Re: struct usage and setup

    Quote Originally Posted by 2kaud View Post
    1) Yes. Use a vector and use STL class string instead of char[]. If you use vector with your current struct then you will need to implement constructor/destructor/copy constructor and operator= functions.
    If all the types in a struct are trivial types (which they are in the OP post, but aren't if he uses std::string), then a copy constructor and assignment operator aren't required, the compiler will automagically provide both. If you don't need the dynamic sizing of a std::string, then sticking to char[] does have some benefits/advantages.

    The same is true for streaming to/from disk. If all types are trivial you can just dump/load the binary contents of the structure into a file (in so far as the structure doesn't change between dump and load). If you use non trivial types, you'll need explicit code to handle streaming.

  7. #7
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    Re: struct usage and setup

    Yeah.. I meant to say 'If you use vector with string then you will need to implement...' but interfaced with keyboard before brain had finished booting-up.

    Personally, I always implement constructors/destructors/assignment etc for all classes/structs as I've seen too many examples of subtle bugs being introduced when a pointer/non-trivial type is sneeked into the class/struct when reliance is placed upon the default versions.
    Last edited by 2kaud; July 5th, 2013 at 08:58 AM.
    All advice is offered in good faith only. You are ultimately responsible for effects of your programs and the integrity of the machines they run on.

  8. #8
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    Re: struct usage and setup

    Quote Originally Posted by OReubens View Post
    If all the types in a struct are trivial types (which they are in the OP post, but aren't if he uses std::string), then a copy constructor and assignment operator aren't required, the compiler will automagically provide both. [...]
    Even then explicitly impementing the rule-of-three members wouldn't strictly be required: The ones the compiler generates by default would properly call the appropriate counterparts on the member instances. Implementing them is only required if the class/struct in question manages its own non-trivial resources.

    Of course, however, serialization would need to be implementex explicitly in this case, as has been said.

    Quote Originally Posted by 2kaud View Post
    Personally, I always implement constructors/destructors/assignment etc for all classes/structs as I've seen too many examples of subtle bugs being introduced when a pointer/non-trivial type is sneeked into the class/struct when reliance is placed upon the default versions.
    I wouldn't agree that this necessarily is a good idea: If your explicit implementations only mimic the behavior of those that would be generated by default by the compiler anyway, then you've got an extra chance to introduce your own bugs, which wouldn't be there if you'd leave that job to the compiler.
    Last edited by Eri523; July 5th, 2013 at 07:18 PM.
    I was thrown out of college for cheating on the metaphysics exam; I looked into the soul of the boy sitting next to me.

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  9. #9
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    Re: struct usage and setup

    If your explicit implementations only mimic the behavior of those that would be generated by default by the compiler anyway, then you've got an extra chance to introduce your own bugs, which wouldn't be there if you'd leave that job to the compiler.
    Ideally, you are correct. Anything which can be left to the compiler should be. But, as I said, I've seen far too many times inexperienced programmers adding a pointer/non-trivial type into a struct/class (or changing a char array into a string or a char pointer) which previously only held trivial types and then wondering why programs which previously worked correctly then start behaving wrongly. They hadn't realised the repercussions of what they had done as there was no copy constructor/operator assignment function coded for them to change. I've wasted hours over trying to find these types of problems. It's now one of the first things I look for when asked to 'sort out' a program that has started to behave weirdly. [Incidentially this turned out to be the cause of my GlobalFree issue post way back in January].
    All advice is offered in good faith only. You are ultimately responsible for effects of your programs and the integrity of the machines they run on.

  10. #10
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    Re: struct usage and setup

    Quote Originally Posted by 2kaud View Post
    Ideally, you are correct. Anything which can be left to the compiler should be. But, as I said, I've seen far too many times inexperienced programmers adding a pointer/non-trivial type into a struct/class (or changing a char array into a string or a char pointer) which previously only held trivial types and then wondering why programs which previously worked correctly then start behaving wrongly. They hadn't realised the repercussions of what they had done as there was no copy constructor/operator assignment function coded for them to change. I've wasted hours over trying to find these types of problems. It's now one of the first things I look for when asked to 'sort out' a program that has started to behave weirdly. [Incidentially this turned out to be the cause of my GlobalFree issue post way back in January].
    Usually these problems arise when development haphazardly mixes 'C' POD style functions that wipe out memory or set memory using things such as memcpy(), memset(), etc. The struct started out as POD, but during the development cycle, someone added a std::string, or any other non-POD type to the struct, making functions like memset() absolutely disastrous.

    That's why I always stress that unless the struct is from a C-based API, beware of using memset(), memcpy(), malloc(), ZeroMemory() etc. on your own structs to set the memory or to allocate memory.

    Regards,

    Paul McKenzie

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