Passing Objects, Constructors, Pointers, References, Values ???

This is a discussion on Passing Objects, Constructors, Pointers, References, Values ??? within the C++ Programming forums, part of the General Programming Boards category; Hello once again my friends. I have a problem here. I cannot understand the use of the constructor,destructor,and copy constructor. ...

  1. #1
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    Question Passing Objects, Constructors, Pointers, References, Values ???

    Hello once again my friends. I have a problem here. I cannot understand the use of the constructor,destructor,and copy constructor. What are these things ? Do I need these ? How am I suppose to know the time they called ?

    This is my first misunderstanding. Let's solve this and then we move on to the next.

  2. #2
    Deathray Engineer MacGyver's Avatar
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    Before you get into constructors, destructors and copy constructors, you really should make sure you know what a class is and why you would use it.

    A constructor is a way to initialize a class. If you have a class which has an integer x, you may want to give x a value when an object of that class is instantiated. That is what a constructor is for.

    A deconstructor is when you want to delete an object. Let's say you have an object which has a file open. If that object is deleted, you want to make sure you close the file, right? If it allocated dynamic memory, you want to make sure that memory is deallocated (ie. free()'ed in C, delete'ed in C++), right? This is what a deconstructor can do. It can deallocate resources upon object destruction (or technically right before it).

    A copy constructor is a bit more detailed, but it's the idea of writing a special constructor to make a copy of itself. By default a "shallow copy" is performed in C++, and sometimes this is not desirable, so a copy constructor is created to perform a "deep copy". You can research this later, but this is not always needed.

  3. #3
    C++まいる!Cをこわせ!
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    In classes, there are three types of special functions:
    Cosntructors - these are called to initialize your objects. They are called when first create your class. Their purpose is to make your class ready to use. Typically like a string constructor can take a const char* pointer to put that string into your string class.
    Copy constructor is called when you try to copy your object, such as obj1 = obj2. Its job is to make sure that the copy is successfully done. The compiler generates a default one if you don't declare one, which copies all member of the class to the class your are copying into. If you need to change this behavior (for example if you have pointers as class members), then you overload this.
    A destructor is called when your object is being destroyed. Its purpose is to clean up and free any memory held by the class.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

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    Ok. So, a Constructor is a special function that initialize the class.

    for example.
    Code:
    class SimpleCat
    {
                public:
                      SimpleCat();   //constructor
                      ~SimpleCat();   //deconstructor
    };
    
    SimpleCat::SimpleCat()
    {
              cout << "This is the constructor of the class..." << endl;
    }
    
    SimpleCat::~SimpleCat()
    {
           cout << This is deconstructor of the class..." << endl;
    }
    So if I try this: SimpleCat Frisky; The result should be ==> This is the constructor of the class...

    How can I delete an object in order to produce the output message of deconstructor ?


    PS:I am not at home, so I cannot compile to see what's the output. Here, there is no programming...it's a PC full of games and antiviruses...Home Home...sweet Home alabama :P lol

  5. #5
    Deathray Engineer MacGyver's Avatar
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    It will be destroyed at the end of the function if it is created on the stack.

    To test constructing and deconstructing objects, I would suggest you use dynamic memory (for a number of reasons).

    Code:
    SimpleCat *c = new SimpleCat();
    cout << "After construction...." << endl;
    delete c;
    cout << "After deletion....." << endl;
    That kind of thing.

  6. #6
    The larch
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    Or create an artificial scope with {}
    Code:
    int main()
    {
      {
        SimpleCat c;
        cout << "After construction...." << endl;
      } //c goes out of scope
      cout << "After deletion....." << endl;
    }
    I might be wrong.

    Thank you, anon. You sure know how to recognize different types of trees from quite a long way away.
    Quoted more than 1000 times (I hope).

  7. #7
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    I compiled this:

    Code:
    #include <iostream>
    
    using namespace std;
    
    class SimpleCat
    {
                public:
                      SimpleCat();   //constructor
                      ~SimpleCat();   //deconstructor
    };
    
    SimpleCat::SimpleCat()
    {
              cout << "This is the constructor of the class..." << endl;
    }
    
    SimpleCat::~SimpleCat()
    {
           cout << "This is deconstructor of the class..." << endl;
    }
    
    int main()
    {
        SimpleCat c;
        cout << "Message after create object c" << endl;
        return 0;
    }
    The output:
    Code:
    This is the constructor of the class...
    Message after create object c
    This is the deconstructor of the class...

    As I thought, when I create the object, the object's class constructor is called. Then, when the main() returns, the deconstructor is automatically called. So, I don't have to delete c object manually. Should I ?

  8. #8
    Deathray Engineer MacGyver's Avatar
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    Manually delete it if you manually allocate it. If it's automatic on the stack as local function variables, no you don't have to delete anything.

    You still need a deconstructor, however, if you need the object to free up resources.

  9. #9
    Frequently Quite Prolix dwks's Avatar
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    Is there such a thing as a deconstructor? Or are they only destructors?
    dwk

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  10. #10
    Deathray Engineer MacGyver's Avatar
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    Yeah, deconstructor.... destructor.... I need to get my terms straight.

  11. #11
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    Quote Originally Posted by BlackSlash12 View Post
    As I thought, when I create the object, the object's class constructor is called. Then, when the main() returns, the deconstructor is automatically called. So, I don't have to delete c object manually. Should I ?
    The general rule is: if you create the object on the stack (SimpleCat myCat) it will be destroyed when the function ends.
    If you create it on the heap using new (SimpleCat* pMyCat = new SimpleCat), then you must explicitly free it using delete (delete pMyCat), otherwise the object will never be destroyed and the destructor will never be called.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  12. #12
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    OK, I got the point. What about copy constructor ? Could you please, give me a similar source code example ?

  13. #13
    C++まいる!Cをこわせ!
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    Bad code.
    Last edited by Elysia; 12-14-2007 at 02:45 PM.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  14. #14
    C++ Witch laserlight's Avatar
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    Eh, actually, that invokes the copy assignment operator. This is an example that invokes the copy constructor:
    Code:
    class MyClass
    {
    public:
    	MyClass(const MyClass& rMyClass); // Copy constructor
    };
    
    int main()
    {
        MyClass a;
        MyClass b(a); // Invokes copy constructor
    }
    Of course, in this case the compiler provided copy constructor will work just fine since... there's nothing to copy.
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  15. #15
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    Quote Originally Posted by Elysia View Post
    Code:
    class MyClass
    {
    public:
    	MyClass(const MyClass& rMyClass); // Copy constructor
    };
    
    int main()
    {
    	MyClass a;
    	MyClass b;
    	a = b; // Invokes copy constructor
    }
    Actually it doesn't. It'll invoke the assignment operator. That's why classes that overload the copy constructor should also overload the assignment operator.

    The copy constructor is called in any case where a copy of the class is needed, except the above.

    On the other hand, this does call the copy constructor:
    Code:
    class MyClass
    {
    public:
    	MyClass(const MyClass& rMyClass); // Copy constructor
    };
    
    int main()
    {
    	MyClass a;
    	MyClass b = a;// b is copy constructed from a.
            MyClass c(a); // alternate syntax for the above. Some say this is less ambiguous.
    }
    EDIT: laserlight beat me to it.
    Last edited by King Mir; 12-14-2007 at 02:47 PM.
    It is too clear and so it is hard to see.
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    He could have cooked his rice much sooner.

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