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    passing structs to functions ?


    hi im having a few problems with this program im making. Its a library program being made in unix g++. When i compile i get 2 errors

    neil.cc:120: conversion from `cust *' to non-scalar type `cust' requested
    neil.cc:123: conversion from `cust *' to non-scalar type `books' requested


    im having problems passing my structs to my functions

    e.g. when 1 is pressed in the menu the member_list function should run and display all memembers of the library database. Im reading this data from a text file.

    can anyone help im really desperate ??

    #include <iostream.h>
    #include <stdlib.h>
    #include <string>
    #include <fstream.h>

    const int maxmembers = 1000;
    const int maxbooks = 2000;

    struct cust
    {
    string surname;
    string house_no;
    string road;
    string ID;
    int books_borrowed;
    };

    struct books
    {
    string author;
    string title;
    string category;
    string ISBN;
    int loaned;
    };

    void member_list(cust c)
    {
    for(int i=0; i< maxbooks;i++)
    {
    cout<< "Surname\t\tHouse No\t\tRoad\t\tID" << endl;
    cout << c.surname[i] << endl;
    cout << c.house_no[i] << endl;
    cout << c.road[i] << endl;
    cout << c.ID[i] << endl;
    }
    }

    void book_list(books b)
    {
    for(int i=0; i< maxbooks;i++)
    {
    cout<< "Author\t\tTitle\t\tCategory\t\tISBN" << endl;
    cout << b.author[i] << endl;
    cout << b.title[i] << endl;
    cout << b.category[i] << endl;
    cout << b.ISBN[i] << endl;
    }
    }

    main()
    {
    char uTest[100];
    int cRead = 0;
    int nTest[100];

    //variables for type people and books
    books book[maxbooks];
    cust user[maxmembers];

    ifstream user_data;
    user_data.open("userdata.txt",ios::in);

    do
    {
    user_data.getline(uTest,100,';');
    user[cRead].surname = uTest;
    user_data.getline(uTest,100,';');
    user[cRead].house_no = uTest;
    user_data.getline(uTest,100,';');
    user[cRead].road = uTest;
    user_data.getline(uTest,100,'\n');
    user[cRead].ID = uTest;

    if (user_data)
    {
    cRead++;
    }

    }while (user_data && cRead < maxmembers);

    user_data.close();

    ifstream book_data;
    book_data.open("bookdata.txt",ios::in);

    do
    {
    book_data.getline(uTest,100,';');
    book[cRead].author = uTest;
    book_data.getline(uTest,100,';');
    book[cRead].title = uTest;
    book_data.getline(uTest,100,';');
    book[cRead].category = uTest;
    book_data.getline(uTest,100,'\n');
    book[cRead].ISBN = uTest;

    if (book_data)
    {
    cRead++;
    }

    }while (book_data && cRead < maxbooks);

    book_data.close();

    int choice;
    cout <<"Library Menu" << endl << endl ;
    cout <<"[1] List Library Members and Member Information" << endl;
    cout <<"[2] List Library Books and Book Information" << endl ;
    cout <<"[3] Issue Library Book" << endl ;
    cout <<"[4] Return Library Book" << endl ;
    cout <<"[5] Exit Library System" << endl ;
    cout <<"Enter Your Required Choice" << endl;
    cin >> choice;

    switch(choice)
    {
    case 1:
    member_list(user);
    break;
    case 2:
    book_list(user);
    break;
    case 5:
    exit(1);
    break;
    default:
    cout << "This is not an option" << endl;
    break;
    }

    }
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    Your problem is here:
    Code:
    void member_list(cust c)
    {
    ...<snip>..
    }
    ...
    ... Some more code
    ....
    main()
    {
    ...
    cust user[maxmembers];
    ...
    ...
    case 1:
       member_list(user);
       break;
    Basically, user is declared as an array (with 2000 elements) of cust, whereas your member_list() function is expecting a single cust as the argument. You can't pass the entire array to the function as an argument, when it is not expecting one. That's what the error message is about. The way I'd fix it is to change the definition of member_list() to read like this:
    Code:
    void member_list(cust *c) {
    ...
    }
    Also, within member_list(), you have some other errors in the code. IMHO it should be c[i].surname, c[i].house_no etc., not c.surname[i], c.house_no[i] etc. You have the same errors in book_list() as well. Hope this helps!
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    one thing that might make things easier instead of passing the struct array through the use of a pointer, is you can pass it this way:

    //in main

    function_name(arrayVariable); //where function name is the fucntion you want, and arrayVariable is the variable you want



    // then in your function you it should be like this

    void function_name(arrayType arrayVariable[] )
    {
    ....
    }

    the [] in the argument list specify that there's an array being passed and then you can use it like the person above me said. both ways work, but this might be a little easier.
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    Neildadon,

    Both of these suggestions amount to the same thing. You cannot push a whole structure onto the stack to pass to a function. You must us a pointer to a structure. For reasons of speed and efficiency, you'd want to use pointers anyway. Pointers are relatively small, but your data structures can be quite large. Complex data structures can also be of variable size, unlike fixed-size pointers.

    Get comfortable with using pointers. You can pass anything using a pointer, and that lets you get away with some amazing stuff.

    I'd highly recommend Kyle Loudon's Mastering Algorithms with C, by O'Rielly & Associates. Once you see some of the amazing things that can be pulled off with pointers, you'll have all the incentive that you need to learn how to use them.
    Clay Dowling
    Lazarus Notes
    Articles and commentary on web development
    http://www.lazarusid.com/notes/
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    thanks for all your help everyone

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