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Full Discussion: Doubts on C++ copy constructor concept
Top Forums Programming Doubts on C++ copy constructor concept Post 302503573 by royalibrahim on Friday 11th of March 2011 01:48:55 AM
Old 03-11-2011
Doubts on C++ copy constructor concept
Hi,

If I run the following program

Code:
class A {
public:
    A()  { cout << "default" << endl; }
    A(const A&) { cout << "copy" << endl; }
};

A tmp; 
A fun() {
    return tmp;
}

A test() {
    A tmp;
    cout << &tmp << endl;
    return tmp;
}

int main() {
    A a = fun();
    A b = test();
    cout << &b << endl;
}

I am getting the following output:

default
copy
default
0xbfbe1792
0xbfbe1792

My doubts are:

1) Why 'const' keyword is so necessary (without which compiler is throwing error) here? why it is not optional and why compiler is stressing to add it which is not happening in normal times?

2) Copy constructor is called when an object is returned by value from a function, as it is one of the place where copy constructor should be called. But, why it is not happening during the second object b's creation?

3) In the above code the output shows the same address on the stack for the 2 objects, but the same code in C using structures instead of classes shows a different address. I know local variables (and objects) inside a function are created in the stack and getting destroyed after the function call. Hence, theoretically it should point to different addresses, with a local copy for each one. But, why in C++, it is holding the same address??
 

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LEARN ABOUT REDHAT

lm

FBB::lm(3bobcat)                                              OFoldStream manipulator                                             FBB::lm(3bobcat)

NAME

       FBB::lm - Manipulator setting left margins of OFoldStream objects

SYNOPSIS

       #include <bobcat/ofoldstream> or
       #include <bobcat/ofoldstreambuf>
       Linking option: -lbobcat

DESCRIPTION

       The lm class implements a manipulator that can be inserted into OFoldStream objects to set the stream's left margin to a requested value.

       Depending  on  the  tab-setting  of  the  OFoldStream  the  inserted value represents the number of blank space characters or the number of
       tab-characters that will be used for the left margin.

       The request will be processed at the next newline character or std::flush or std::endl manipulator that is inserted into the stream.  If  a
       line is still once an lm object and a flush manipulator are inserted into the stream then the new left margin will be effective at the next
       word inserted into that line (cf., the example section below).

       A bad_cast exception is thrown when the manipulator is inserted into an ostream that is not using a OFoldStreambuf buffer.

NAMESPACE

       FBB
       All constructors, members, operators and manipulators, mentioned in this man-page, are defined in the namespace FBB.

INHERITS FROM

       -

CONSTRUCTOR

       o      lm(size_t newValue):
              The standard copy constructor is available.

MEMBER FUNCTIONS

       There are no public or protected member functions in this class.

EXAMPLE

           #include <iostream>
           #include <bobcat/ofoldstream>

           using namespace std;
           using namespace FBB;

           int main()
           {
               OFoldStream out(cout, 0, 80);
               out << "hello world (left margin is 0)" << lm(4) << "
"
                      "this uses a 4 character wide left margin
" <<
                      lm(0) << flush << "at column 0 again
";
               return 0;
           }

FILES

       bobcat/lm - defines the class interface

SEE ALSO

       bobcat(7), manipulators(3bobcat), mlm(3bobcat), ofoldstream(3bobcat)

BUGS

       None Reported.

DISTRIBUTION FILES

       o      bobcat_3.01.00-x.dsc: detached signature;

       o      bobcat_3.01.00-x.tar.gz: source archive;

       o      bobcat_3.01.00-x_i386.changes: change log;

       o      libbobcat1_3.01.00-x_*.deb: debian package holding the libraries;

       o      libbobcat1-dev_3.01.00-x_*.deb: debian package holding the libraries, headers and manual pages;

       o      http://sourceforge.net/projects/bobcat: public archive location;

BOBCAT

       Bobcat is an acronym of `Brokken's Own Base Classes And Templates'.

COPYRIGHT

       This is free software, distributed under the terms of the GNU General Public License (GPL).

AUTHOR

       Frank B. Brokken (f.b.brokken@rug.nl).

libbobcat1-dev_3.01.00-x.tar.gz                                      2005-2012                                                    FBB::lm(3bobcat)
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