Copy Constructors... *Newbie Alert!*

This is a discussion on Copy Constructors... *Newbie Alert!* within the C++ Programming forums, part of the General Programming Boards category; When do they get called?...

  1. #1
    PLC
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    Copy Constructors... *Newbie Alert!*

    When do they get called?

  2. #2
    Registered User Codeplug's Avatar
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    Write some test code and see...

    gg

  3. #3
    PLC
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    When does this copy constructor get called? and why..

    Code:
    #include <iostream.h>
    #include <iomanip.h>
    
    int linecount = 1;
    char label = 'A';
    
    class Data
    {
        char ID;
        int value;
    public:
    
        // Default constructor
        Data()
        {
           ID = label++;
           value = 0;
           cout << setw(2) << linecount++ 
              << ": Default Constructor " << ID << " " << value << endl; 
        }
    
        // Parametrized Constructor
        Data(int v)
        {
           ID = label++;
           value = v;
           cout << setw(2) << linecount++ 
              << ": Parametrized Constructor " << ID << " " << value << endl; 
        }
    
        // Copy Constructor
        Data(const Data &d)
        {
           // copy only the value, we get a unique ID
           ID = label++;
           value = d.value;
           cout << setw(2) << linecount++ 
              << ": Copy Constructor " << ID << " " << value 
              << " From " << d.ID << " " << d.value << endl; 
        }
    
      // Assignment Operator
      Data &operator=(Data &d)
      {
        // copy only the value
        value = d.value;
        cout << setw(2) << linecount++
             << ": Assignment of " << ID << " " << value
             << " From " << d.ID << " " << d.value << endl;
        return *this;
      }
    
    
        // Destructor
        ~Data()
        {
           cout << setw(2) << linecount++ 
              << ": Destructor " << ID << " " << value << endl; 
        }
    };
    
    
    // A global function which uses Data objects
    Data Mangle(Data d)
    {
         Data temp = d;
         return temp;
    }
    
    
    // The main program.
    int main()
    {
       Data a(10);
       Data b = a;
       Data c;
    
       c = Mangle(a);
       return 0;
    }

  4. #4
    PLC
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    hmmm

    Data b = a;

    it copies the members of A into the "this" object.. which is b. Thus the copy constructor is called. oic

    but why isnt the assignment operator called..

  5. #5
    PLC
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    hahah.. n/m got it.


    thx for the help.

  6. #6
    Cat without Hat CornedBee's Avatar
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    Initialization is translated to a copy construction even if in the syntax
    type name = existing;
    This line has the same effect as
    type name(existing);
    All the buzzt!
    CornedBee

    "There is not now, nor has there ever been, nor will there ever be, any programming language in which it is the least bit difficult to write bad code."
    - Flon's Law

  7. #7
    Skunkmeister Stoned_Coder's Avatar
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    Initialization is translated to a copy construction even if in the syntax
    type name = existing;
    This line has the same effect as
    type name(existing);
    What if the copy constructor was explicit?
    Free the weed!! Class B to class C is not good enough!!
    And the FAQ is here :- http://faq.cprogramming.com/cgi-bin/smartfaq.cgi

  8. #8
    Cat without Hat CornedBee's Avatar
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    Can it be?
    All the buzzt!
    CornedBee

    "There is not now, nor has there ever been, nor will there ever be, any programming language in which it is the least bit difficult to write bad code."
    - Flon's Law

  9. #9
    Skunkmeister Stoned_Coder's Avatar
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    oh yes.... look
    Code:
    class A
    {
       public:
         explicit A(const A&) {//impl}
       private:
         //members
    };
    
    int main()
    {
       A obj;
       A copy(obj); // fine direct call of cc
       A copy2 = obj; // error!
       return 0;
    }
    Free the weed!! Class B to class C is not good enough!!
    And the FAQ is here :- http://faq.cprogramming.com/cgi-bin/smartfaq.cgi

  10. #10
    Cat without Hat CornedBee's Avatar
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    But if it is, can it still be used for arg-by-value? That's an implicit call after all.
    All the buzzt!
    CornedBee

    "There is not now, nor has there ever been, nor will there ever be, any programming language in which it is the least bit difficult to write bad code."
    - Flon's Law

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