Passing objects to functions

This is a discussion on Passing objects to functions within the C++ Programming forums, part of the General Programming Boards category; Hello, i have a question, so im reading a section on classes and pointers and its telling that we can ...

  1. #1
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    Post Passing objects to functions

    Hello, i have a question, so im reading a section on classes and pointers and its telling that we can modify a object using :

    <return type> <function name>(<class name> *<name of pointer>)

    so when they say modify the object, do they mean that you can modify the member variables of the object of a class?

    for example:
    if i had a class named Tires and in the class Tires, i had a member variable named PSI, could i pass the object to a function and change the value of PSI?

  2. #2
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    Yes.

    An object (an instance of a class, which is like a blueprint) consists of its fields (member variables). Fields are the only things you can change about an object.

  3. #3
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    Quote Originally Posted by cyberfish View Post
    Yes.

    An object (an instance of a class, which is like a blueprint) consists of its fields (member variables). Fields are the only things you can change about an object.
    i still dont get how you would set it up, could you give me a example please?

  4. #4
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    Not tested.
    Code:
    class Tires
    {
    public:
       Tires() : psi_(0) {}
       void setPSI(const int psi) { psi_= psi; }
       int getPSI() const { return psi_; }
    
    private:
       int psi_;
    };
    
    void SetTirePsiTo30(Tires *t)
    {
        t->setPSI(30);
    }
    
    int main(void)
    {
        Tires t;
    
        // Because the argument is a pointer, we must pass the address of the variable.
        SetTirePsiTo30(&t);
        return 0;
    }

  5. #5
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    is this correct??
    Code:
    #include <iostream>
    using namespace std;
    
    class Tires
    {
       public:
        int PSI;
        float width;
        int weight;
    
    
        void ChangePSI(Tires *change)
        {
        change->PSI = 100;
        }
    };
    int main()
    {
        Tires *ptrTires;
    
        ptrTires = new Tires;
    
        cout << "how much psi does your tire have?" << endl;
        cin >> ptrTires->PSI;
    
        ChangePSI(ptrTires);
    
        cout << ptrTires->PSI << endl;
        return 0;
    
    }

  6. #6
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    ChangePSI won't be a member function then.

    But otherwise yes, you can do that.

    Although it's more natural to make it a member function, and do

    ptrTires->ChangePSI(100);

  7. #7
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    Quote Originally Posted by cyberfish View Post
    ChangePSI won't be a member function then.

    But otherwise yes, you can do that.

    Although it's more natural to make it a member function, and do

    ptrTires->ChangePSI(100);
    what do you mean ChangePSI wont be a member function??,i dont get what ptrTires->ChangePSI(100) would do

  8. #8
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    Forget about pointers for now.

    Actually, forget about C++, too.

    Learn about general OOP concepts first would make your life much easier.

    The best I could find was the first page of Java's tutorial
    What Is an Object? (The Java™ Tutorials > Learning the Java Language > Object-Oriented Programming Concepts)
    What Is a Class? (The Java™ Tutorials > Learning the Java Language > Object-Oriented Programming Concepts)

    Don't worry about inheritance for now.

    The second link has some Java code, but it should be pretty self explanatory.

  9. #9
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    Quote Originally Posted by cyberfish View Post
    Forget about pointers for now.

    Actually, forget about C++, too.

    Learn about general OOP concepts first would make your life much easier.

    The best I could find was the first page of Java's tutorial
    What Is an Object? (The Java™ Tutorials > Learning the Java Language > Object-Oriented Programming Concepts)
    What Is a Class? (The Java™ Tutorials > Learning the Java Language > Object-Oriented Programming Concepts)

    Don't worry about inheritance for now.

    The second link has some Java code, but it should be pretty self explanatory.
    I need the pointer though because without it i would only be changing the parameter, i want to change the actual value all together.

  10. #10
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    Well, what I am suggesting is, if you learn about general OOP concepts first, this problem will be very simple.

    ptrTires->ChangePSI(100); calls the ChangePSI function of the ptrTires object, which modifies the object's state.

    But that won't make sense until you learn about OOP.

  11. #11
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    Quote Originally Posted by cyberfish View Post
    Well, what I am suggesting is, if you learn about general OOP concepts first, this problem will be very simple.

    ptrTires->ChangePSI(100); calls the ChangePSI function of the ptrTires object, which modifies the object's state.

    But that won't make sense until you learn about OOP.
    Okay, thanks for the advice and links! and your time

  12. #12
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    Code:
    #include <iostream>
    using namespace std;
    
    class Waterbottle
    {
       public:
        string Brand;
        float height;
        int WaterLevelPercent;
    
        void Waterbottle::DrinksWater(Waterbottle *bottle)
        {
            bottle->WaterLevelPercent= bottle->WaterLevelPercent - 1;
        }
    };
    
    int main()
    {
        Waterbottle *ptrBottle;
        ptrBottle = new Waterbottle;
    
        ptrBottle->WaterLevelPercent = 100;
        DrinksWater(ptrBottle);
    
        cout << DrinksWater->WaterLevelPercent << endl;
    
        return 0;
    }
    i tried making this to implement my skills with classes but, i get a error where it says that DrinksWater is not declared but, i declared it as a member function in the class definition

  13. #13
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    The classname::methodname syntax is for implementing methods outside the class definition.

    If you define the method within the class definition (called an inline definition), you can just say "void DrinksWater".

    Also, you don't need the "bottle" parameter, because that is implicit.
    Code:
    class Waterbottle
    {
    ...
        void DrinksWater() { WaterLevelPercent--; }
    }
    It decreases the WaterLevelPercent of "this" Waterbottle.

    The method (action) is associated with "this" Waterbottle, and you can't use it to modify any other Waterbottle.

  14. #14
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    Code:
    #include <iostream>
    using namespace std;
    
    class Waterbottle
    {
       public:
        string Brand;
        float height;
        int WaterLevelPercent;
    
        void DrinksWater()
        {
           WaterLevelPercent--;
        }
    };
    
    int main()
    {
        Waterbottle CrystalGeyser;
    
        DrinksWater();
    
        cout << CrystalGeyser.WaterLevelPercent << endl;
    
        return 0;
    }
    i tried doing this, but, i still got the undeclared error

  15. #15
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    That's because you can't just call DrinksWater.

    What if you have many Waterbottles?

    Which waterbottle will that drink from?

    To tell it to drink from CystalGeyser, do GrystalGeyser.DrinksWater();

    It's just like how you can't do

    cout << WaterLevelPercent << endl;

    WaterLevelPercent of WHICH bottle?

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