Doubts on C++ copy constructor concept Post: 302503573

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Top Forums Programming Doubts on C++ copy constructor concept Post 302503573 by royalibrahim on Friday 11th of March 2011 01:48:55 AM

03-11-2011

Registered User

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??

royalibrahim

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LEARN ABOUT DEBIAN

tau_get_event_vals

TAU_GET_EVENT_VALS(3)					      TAU Instrumentation API					     TAU_GET_EVENT_VALS(3)

NAME

       TAU_GET_EVENT_VALS - Gets user event data for given user events.

SYNOPSIS

       C/C++:

       TAU_GET_EVENT_VALS(const char **inUserEvents, int numUserEvents, int **numEvents, double **max, double **min, double **mean,
			  double **sumSqe);

DESCRIPTION

       Retrieves user defined event data for the specified user defined events. The list of events are specified by the first parameter
       (eventList) and the user specifies the number of events in the second parameter (numUserEvents). TAU returns the number of times the event
       was invoked in the numUserEvents. The max, min, mean values are returned in the following parameters. TAU computes the sum of squares of
       the given event and returns this value in the next argument (sumSqe).

EXAMPLE

       C/C++ :

	 const char **eventList;
	 int numEvents;

	 TAU_GET_EVENT_NAMES(eventList, numEvents);

	 cout << "numEvents: " << numEvents << endl;

	 if (numEvents > 0) {
	   int *numSamples;
	   double *max;
	   double *min;
	   double *mean;
	   double *sumSqr;

	   TAU_GET_EVENT_VALS(eventList, numEvents, numSamples,
	     max, min, mean, sumSqr);
	   for (int i=0; i<numEvents; i++) {
	     cout << "-------------------
";
	     cout << "User Event:	 " << eventList[i] << endl;
	     cout << "Number of Samples: " << numSamples[i] << endl;
	     cout << "Maximum Value:	 " << max[i] << endl;
	     cout << "Minimum Value:	 " << min[i] << endl;
	     cout << "Mean Value:	 " << mean[i] << endl;
	     cout << "Sum Squared:	 " << sumSqr[i] << endl;
	   }
	 }
       }

SEE ALSO

       TAU_REGISTER_EVENT(3), TAU_REGISTER_CONTEXT_EVENT(3), TAU_GET_EVENT_NAMES(3)

								    08/31/2005						     TAU_GET_EVENT_VALS(3)

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tau_get_event_vals