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Who is doing the coping? [implicit-copy | deep-copy | pick constructor] [message #38422]

Sat, 15 December 2012 05:24

navi
Messages: 107
Registered: February 2012
Location: Sydney, Australia

Experienced Member

#include <CtrlLib/CtrlLib.h>

using namespace Upp;

class myobject: public Moveable<myobject>{
	public:
	String a;
	String b;
	String c;
};

GUI_APP_MAIN
{
	// gui code

	TopWindow w;
	LineEdit e;
	
	e.HSizePos().VSizePos();
	w.SetRect(0,0,400,600);
	w<<e;

	// codes for test example

	Vector<myobject> v;
	myobject tmp;
	String s;
	
	for(int i=0; i<5; i++){
		tmp.a=Format("a value=%d",i);
		tmp.b=Format("b value=%d",i);
		tmp.c=Format("c value=%d",i);
		
		v.Add(tmp);
		
		s<<"\nObject:"<<i<<"\n";
		s<<tmp.a<<",\n"<<tmp.b<<",\n"<<tmp.c<<"\n";		
		
	}

	e.Set(s);

	w.Run();
}

http://www.ultimatepp.org/src$Core$Vector$en-us.html

class Vector : public MoveableAndDeepCopyOption< Vector<T> >

has 3 Add method,

1. T& Add(): Adds new default constructed element to Vector.

2. T& Add(const T& x) : Requires T to have deep copy constructor.

3. T& AddPick(pick_ T& x) : Requires T to have pick constructor.

In above code 'myobject' does not have any pick constructor, or deep copy constructor. As on the example code, if 'tmp' object is added to a vector using Add(T& x) method, then 'tmp' gets nicely copied into the new vector element. without nullifying or destructing the 'tmp' object. since I did not write a deep copy constructor, so:

* who/what is doing the coping?
* which contractor is getting called? (if any!?)

I have tried reading the followings but still can not understand why is the above code working?

however, after reading this: http://www.cplusplus.com/articles/y8hv0pDG/

I concluded that C++ is writing a implicit copy constructor which is doing a member-wise copy of 'myobject' and neither U++ deep copy nor pick is being used. since myobject only has 3 String and String class has operator= overloaded, so implicit copy constructor is able to carryout its member-wise copy. Is this correct?

[Updated on: Sat, 15 December 2012 06:01]

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