Question about structs in class which wouldn't save value.
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Thread: Question about structs in class which wouldn't save value.

  1. #1
    Join Date
    Apr 2010
    Posts
    26

    Question about structs in class which wouldn't save value.

    I have a question about this particular code.
    The structs would not save the value entered by the user into its class' attributes.
    If you do not understand what i am talking about, please compile the code below. Can someone kindly explain what is the problem and how to fix it? Thank you for your time.

    Code:
    #include <iostream>
    #include <string>
    
    using namespace std;
    
    class Sam
    {
        struct sample{
            int a;
            string b;
            };
    
        public:
    
            void set_a();
            void set_b();
            int ret_a();
            string ret_b();
    };
    
    
    void Sam::set_a()
    {
        int a1;
        cout<<"Insert int a";
        cin>>a1;
        sample().a=a1;
    }
    
    int Sam::ret_a()
    {
        return sample().a;
    }
    
    void Sam::set_b()
    {
        string b1;
        cout<<"Insert string b";
        cin>>b1;
        sample().b=b1;
    }
    
    string Sam::ret_b()
    {
        return sample().b;
    }
    
    int main()
    {
        Sam c;
    
        c.set_a();
        c.set_b();
    
        cout<<c.ret_a()<<endl<<c.ret_b();  //It should output the same value inputted by the user             
                                                                 // but it shows nonsense. why is it so?
        return 0;
    }

  2. #2
    Join Date
    Apr 1999
    Posts
    27,426

    Re: Question about structs in class which wouldn't save value.

    Quote Originally Posted by hayloiuy View Post
    Can someone kindly explain what is the problem
    What does this line do?
    Code:
    sample().a=a1;
    It creates a temporary variable of type "sample". Temporary variables are destroyed as soon as the statement is executed, therefore nothing has happened.
    and how to fix it?
    You create a member variable.
    Code:
    #include <iostream>
    #include <string>
    
    using namespace std;
    
    class Sam
    {
        struct sample{
            int a;
            string b;
            };
         
        sample mySampleA, mySampleB;
        public:
    
            void set_a();
            void set_b();
            int ret_a();
            string ret_b();
    };
    
    
    void Sam::set_a()
    {
        int a1;
        cout<<"Insert int a";
        cin>>a1;
        mySampleA.a=a1;
    }
    
    int Sam::ret_a()
    {
        return mySampleA.a;
    }
    Regards,

    Paul McKenzie

  3. #3
    Join Date
    Apr 2010
    Posts
    26

    Re: Question about structs in class which wouldn't save value.

    AARGGGHHHH this is just a very very simple mistake which i overlook. I spent 2 hours thinking what is the problem. Imagine that. If there are any thumbs up in this forum i will give you a hundred. Thank you very much and i am very very deeply sorry for wasting your time.

  4. #4
    Join Date
    Oct 2006
    Location
    Sweden
    Posts
    3,632

    Re: Question about structs in class which wouldn't save value.

    If there are any thumbs up
    There is... just add :thumb:
    Debugging is twice as hard as writing the code in the first place.
    Therefore, if you write the code as cleverly as possible, you are, by
    definition, not smart enough to debug it.
    - Brian W. Kernighan

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    http://www.codeguru.com/forum/announ...nouncementid=6
    and http://www.codeguru.com/forum/showthread.php?t=366302 before posting

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  5. #5
    Join Date
    Nov 2006
    Location
    Essen, Germany
    Posts
    1,344

    Re: Question about structs in class which wouldn't save value.

    Since copying a std::string object might be expensive changing the ret_b() method to

    Code:
    const std::string& Sam::ret_b() const
    {
       return b;
    }
    might be useful, too.

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