template<typena me _Tp, typename _Alloc>
void
vector<_Tp, _Alloc>::
_M_insert_aux(i terator __position, const _Tp& __x)
{
if (this->_M_impl._M_fin ish != this->_M_impl._M_end _of_storage)
{
this->_M_impl.constr uct(this->_M_impl._M_fin ish,
*(this->_M_impl._M_fin ish - 1));
++this->_M_impl._M_fin ish;
_Tp __x_copy = __x;
std::copy_backw ard(__position,
iterator(this->_M_impl._M_fin ish-2),
iterator(this->_M_impl._M_fin ish-1));
*__position = __x_copy;
}
else ...
I had a couple of doubts with regard to code pasted here from
stl_vector.h
a) I was curious why a copy of x is created in the statement:
_Tp __x_copy = __x;
and that is assigned to *position, rather than directly doing
*position = __x ;
b) is it necessary to increment finish ++this->_M_impl._M_fin ish
before the copy_backward.
operation, is there anything wrong in doing the copy backwards first
like this
std::copy_backw ard(__position,
iterator(this->_M_impl._M_fin ish-1),
iterator(this->_M_impl._M_fin ish));
and then
++this->_M_impl._M_fin ish;
Thx
Digz
7  5716 
digz wrote:
:: template<typena me _Tp, typename _Alloc>
:: void
:: vector<_Tp, _Alloc>::
:: _M_insert_aux(i terator __position, const _Tp& __x)
:: {
:: if (this->_M_impl._M_fin ish != this->_M_impl._M_end _of_storage)
:: {
:: this->_M_impl.constr uct(this->_M_impl._M_fin ish,
:: *(this->_M_impl._M_fin ish - 1));
:: ++this->_M_impl._M_fin ish;
:: _Tp __x_copy = __x;
:: std::copy_backw ard(__position,
:: iterator(this->_M_impl._M_fin ish-2),
:: iterator(this->_M_impl._M_fin ish-1));
:: *__position = __x_copy;
:: }
:: else ...
::
:: I had a couple of doubts with regard to code pasted here from
:: stl_vector.h
:: a) I was curious why a copy of x is created in the statement:
:: _Tp __x_copy = __x;
:: and that is assigned to *position, rather than directly doing
:: *position = __x ;
It could be that __x is a reference to an element already in the vector. In
that case you have to save the value, before doing the copy_backward() .
It must handle code like:
v.insert(v.begi n(), v[5]);
::
:: b) is it necessary to increment finish ++this->_M_impl._M_fin ish
:: before the copy_backward.
:: operation, is there anything wrong in doing the copy backwards first
:: like this
::
:: std::copy_backw ard(__position,
:: iterator(this->_M_impl._M_fin ish-1),
:: iterator(this->_M_impl._M_fin ish));
:: and then
:: ++this->_M_impl._M_fin ish;
::
Yes. The increment is connected to the construction of a new object:
this->_M_impl.constr uct(this->_M_impl._M_fin ish,
*(this->_M_impl._M_fin ish - 1));
++this->_M_impl._M_fin ish;
This adds a new object to the vector, so it must also add 1 to the size of
the vector (by incrementing the end pointer).
If it didn't, and something bad happened in copy_backward, the newly created
object could be left created but never destroyed.
Bo Persson
On 2007-04-08 02:07, digz wrote:
template<typena me _Tp, typename _Alloc>
void
vector<_Tp, _Alloc>::
_M_insert_aux(i terator __position, const _Tp& __x)
{
if (this->_M_impl._M_fin ish != this->_M_impl._M_end _of_storage)
{
this->_M_impl.constr uct(this->_M_impl._M_fin ish,
*(this->_M_impl._M_fin ish - 1));
++this->_M_impl._M_fin ish;
_Tp __x_copy = __x;
std::copy_backw ard(__position,
iterator(this->_M_impl._M_fin ish-2),
iterator(this->_M_impl._M_fin ish-1));
*__position = __x_copy;
}
else ...
I had a couple of doubts with regard to code pasted here from
stl_vector.h
a) I was curious why a copy of x is created in the statement:
_Tp __x_copy = __x;
and that is assigned to *position, rather than directly doing
*position = __x ;
A copy have to be made before copy_backwards( ) is called in case the
copy-constructor would throw an exception or we would be left with a
inconsistent vector.
--
Erik Wikström
In article <57************ *@mid.individua l.net>,
"Bo Persson" <bo*@gmb.dkwrot e:
digz wrote:
:: template<typena me _Tp, typename _Alloc>
:: void
:: vector<_Tp, _Alloc>::
:: _M_insert_aux(i terator __position, const _Tp& __x)
:: {
:: if (this->_M_impl._M_fin ish != this->_M_impl._M_end _of_storage)
:: {
:: this->_M_impl.constr uct(this->_M_impl._M_fin ish,
:: *(this->_M_impl._M_fin ish - 1));
:: ++this->_M_impl._M_fin ish;
:: _Tp __x_copy = __x;
:: std::copy_backw ard(__position,
:: iterator(this->_M_impl._M_fin ish-2),
:: iterator(this->_M_impl._M_fin ish-1));
:: *__position = __x_copy;
:: }
:: else ...
::
:: I had a couple of doubts with regard to code pasted here from
:: stl_vector.h
:: a) I was curious why a copy of x is created in the statement:
:: _Tp __x_copy = __x;
:: and that is assigned to *position, rather than directly doing
:: *position = __x ;
It could be that __x is a reference to an element already in the vector. In
that case you have to save the value, before doing the copy_backward() .
It must handle code like:
v.insert(v.begi n(), v[5]);
Yes. But this is a poor (imho) implementation of how to handle that
problem. The copy constructor of _Tp may be arbitrarily expensive and
you want to try and minimize calls to a function of unknown expense.
It is cheap and easy to detect whether __x is self referencing or not.
And if it self referencing, it is also cheap and easy to compute its new
location after shuffling things around.
-Howard
On Apr 8, 12:17 pm, Howard Hinnant <howard.hinn... @gmail.comwrote :
In article <57rsrkF2d4fn.. .@mid.individua l.net>,
"Bo Persson" <b...@gmb.dkwro te:
digz wrote:
:: template<typena me _Tp, typename _Alloc>
:: void
:: vector<_Tp, _Alloc>::
:: _M_insert_aux(i terator __position, const _Tp& __x)
:: {
:: if (this->_M_impl._M_fin ish != this->_M_impl._M_end _of_storage)
:: {
:: this->_M_impl.constr uct(this->_M_impl._M_fin ish,
:: *(this->_M_impl._M_fin ish - 1));
:: ++this->_M_impl._M_fin ish;
:: _Tp __x_copy = __x;
:: std::copy_backw ard(__position,
:: iterator(this->_M_impl._M_fin ish-2),
:: iterator(this->_M_impl._M_fin ish-1));
:: *__position = __x_copy;
:: }
:: else ...
::
:: I had a couple of doubts with regard to code pasted here from
:: stl_vector.h
:: a) I was curious why a copy of x is created in the statement:
:: _Tp __x_copy = __x;
:: and that is assigned to *position, rather than directly doing
:: *position = __x ;
It could be that __x is a reference to an element already in the vector. In
that case you have to save the value, before doing the copy_backward() .
It must handle code like:
v.insert(v.begi n(), v[5]);
Yes. But this is a poor (imho) implementation of how to handle that
problem. The copy constructor of _Tp may be arbitrarily expensive and
you want to try and minimize calls to a function of unknown expense.
It is cheap and easy to detect whether __x is self referencing or not.
And if it self referencing, it is also cheap and easy to compute its new
location after shuffling things around.
-Howard
But I copied this block of code from STL shipped with Suse-
Linux-10.0 ,
Are you saying that the std::vector code is not optimal enough.What
could be a better/cheaper implementation
for the problem the block of code above is trying to address ??
--Digz
In article <11************ **********@e65g 2000hsc.googleg roups.com>,
"digz" <Di********@gma il.comwrote:
On Apr 8, 12:17 pm, Howard Hinnant <howard.hinn... @gmail.comwrote :
In article <57rsrkF2d4fn.. .@mid.individua l.net>,
"Bo Persson" <b...@gmb.dkwro te:
digz wrote:
:: template<typena me _Tp, typename _Alloc>
:: void
:: vector<_Tp, _Alloc>::
:: _M_insert_aux(i terator __position, const _Tp& __x)
:: {
:: if (this->_M_impl._M_fin ish != this->_M_impl._M_end _of_storage)
:: {
:: this->_M_impl.constr uct(this->_M_impl._M_fin ish,
:: *(this->_M_impl._M_fin ish - 1));
:: ++this->_M_impl._M_fin ish;
:: _Tp __x_copy = __x;
:: std::copy_backw ard(__position,
:: iterator(this->_M_impl._M_fin ish-2),
:: iterator(this->_M_impl._M_fin ish-1));
:: *__position = __x_copy;
:: }
:: else ...
::
:: I had a couple of doubts with regard to code pasted here from
:: stl_vector.h
:: a) I was curious why a copy of x is created in the statement:
:: _Tp __x_copy = __x;
:: and that is assigned to *position, rather than directly doing
:: *position = __x ;
It could be that __x is a reference to an element already in the vector.
In
that case you have to save the value, before doing the copy_backward() .
It must handle code like:
v.insert(v.begi n(), v[5]);
Yes. But this is a poor (imho) implementation of how to handle that
problem. The copy constructor of _Tp may be arbitrarily expensive and
you want to try and minimize calls to a function of unknown expense.
It is cheap and easy to detect whether __x is self referencing or not.
And if it self referencing, it is also cheap and easy to compute its new
location after shuffling things around.
-Howard
But I copied this block of code from STL shipped with Suse-
Linux-10.0 ,
Are you saying that the std::vector code is not optimal enough.
I'm saying *this* implementation of std::vector is not optimal. I've
seen better (I've written better).
What
could be a better/cheaper implementation
for the problem the block of code above is trying to address ??
It would be better to detect the self reference by comparing the address
of __x with the address of the insertion point and the address of the
end of the buffer. If the address of __x lies within that range, it is
self referencing. And in this case, the address of __x is going to
change during the reshuffling to make room for the new copy of __x.
Computing the final address of __x after the reshuffle is trivial: it
is going to move in the buffer by one position.
-Howard
On Apr 11, 9:11 am, Howard Hinnant <howard.hinn... @gmail.comwrote :
In article <1176292518.653 052.110...@e65g 2000hsc.googleg roups.com>,
"digz" <Digvijo...@gma il.comwrote:
On Apr 8, 12:17 pm, Howard Hinnant <howard.hinn... @gmail.comwrote :
In article <57rsrkF2d4fn.. .@mid.individua l.net>,
"Bo Persson" <b...@gmb.dkwro te:
digz wrote:
:: template<typena me _Tp, typename _Alloc>
:: void
:: vector<_Tp, _Alloc>::
:: _M_insert_aux(i terator __position, const _Tp& __x)
:: {
:: if (this->_M_impl._M_fin ish != this->_M_impl._M_end _of_storage)
:: {
:: this->_M_impl.constr uct(this->_M_impl._M_fin ish,
:: *(this->_M_impl._M_fin ish - 1));
:: ++this->_M_impl._M_fin ish;
:: _Tp __x_copy = __x;
:: std::copy_backw ard(__position,
:: iterator(this->_M_impl._M_fin ish-2),
:: iterator(this->_M_impl._M_fin ish-1));
:: *__position = __x_copy;
:: }
:: else ...
::
:: I had a couple of doubts with regard to code pasted here from
:: stl_vector.h
:: a) I was curious why a copy of x is created in the statement:
:: _Tp __x_copy = __x;
:: and that is assigned to *position, rather than directly doing
:: *position = __x ;
It could be that __x is a reference to an element already in the vector.
In
that case you have to save the value, before doing the copy_backward() .
It must handle code like:
v.insert(v.begi n(), v[5]);
Yes. But this is a poor (imho) implementation of how to handle that
problem. The copy constructor of _Tp may be arbitrarily expensive and
you want to try and minimize calls to a function of unknown expense.
It is cheap and easy to detect whether __x is self referencing or not.
And if it self referencing, it is also cheap and easy to compute its new
location after shuffling things around.
-Howard
But I copied this block of code from STL shipped with Suse-
Linux-10.0 ,
Are you saying that the std::vector code is not optimal enough.
I'm saying *this* implementation of std::vector is not optimal. I've
seen better (I've written better).
What
could be a better/cheaper implementation
for the problem the block of code above is trying to address ??
It would be better to detect the self reference by comparing the address
of __x with the address of the insertion point and the address of the
end of the buffer. If the address of __x lies within that range, it is
self referencing. And in this case, the address of __x is going to
change during the reshuffling to make room for the new copy of __x.
Computing the final address of __x after the reshuffle is trivial: it
is going to move in the buffer by one position.
-Howard
Thanks a lot for clarifying this.
This is libstdc++-3.3 , g++-4.0.1 on x86_64-pc-linux-gnu
Is gcc-help the right mailing list to follow up on opensource libstdc+
+ implementations ??
coz after your explanation I cannot imagine how g++ developers are
producing this code and shipping
it with RedHat, Debian, Suse..etc..all Linux Vendors have the same
STL, and on a bad day
a super slow copy constructor can make std::vector really suffer !!
--Digz
In article <11************ **********@q75g 2000hsh.googleg roups.com>,
"digz" <Di********@gma il.comwrote:
This is libstdc++-3.3 , g++-4.0.1 on x86_64-pc-linux-gnu
Is gcc-help the right mailing list to follow up on opensource libstdc+
+ implementations ??
coz after your explanation I cannot imagine how g++ developers are
producing this code and shipping
it with RedHat, Debian, Suse..etc..all Linux Vendors have the same
STL, and on a bad day
a super slow copy constructor can make std::vector really suffer !!
Sorry for the slow response. Yes, gcc-help or libstdc++ would be good
followup lists.
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