std::vector: reserve required?

[Mike -- Email Ignored]

In std::vector, is reserve or resize required?

On:
Linux mbrc32 2.6.22.1-41.fc7 #1 SMP Fri
Jul 27 18:10:34 EDT 2007 i686 athlon
i386 GNU/Linux
Using:
g++ (GCC) 4.1.2 20070502 (Red Hat 4.1.2-12)

The program below fails, but if the reserve(en)
is uncommented, it works. Is this as expected?

// vectst.cc 07/04/08

#include <iostream>
#include <vector>
using namespace std;

int main(int argc, const char* argv[])
{
int en = 10;
vector<int vec;

// vec.reserve(en) ;
for (int jj = 0; jj < en; ++jj)
vec[jj] = jj;
for (int jj = 0; jj < en; ++jj)
cout << vec[jj] << endl;

exit (0);
}

Thanks,
Mike.

[=?ISO-8859-1?Q?Dar=EDo_Griffo?=]

Re: std::vector: reserve required?


Mike -- Email Ignored wrote:

In std::vector, is reserve or resize required?
>
On:
Linux mbrc32 2.6.22.1-41.fc7 #1 SMP Fri
Jul 27 18:10:34 EDT 2007 i686 athlon
i386 GNU/Linux
Using:
g++ (GCC) 4.1.2 20070502 (Red Hat 4.1.2-12)
>
The program below fails, but if the reserve(en)
is uncommented, it works. Is this as expected?
>
// vectst.cc 07/04/08
>
#include <iostream>
#include <vector>
using namespace std;
>
int main(int argc, const char* argv[])
{
int en = 10;
vector<int vec;
>
// vec.reserve(en) ;
for (int jj = 0; jj < en; ++jj)
vec[jj] = jj;
for (int jj = 0; jj < en; ++jj)
cout << vec[jj] << endl;
>
exit (0);
}
>
Thanks,
Mike.

No, you could do that (use reserve), or, use push_back()

int main(int argc, const char* argv[])
{
int en = 10;
vector<int vec;

for (int jj = 0; jj < en; ++jj)
vec.push_back(j j);
for (int jj = 0; jj < en; ++jj)
cout << vec[jj] << endl;

return 0;
}

[Kai-Uwe Bux]

Re: std::vector: reserve required?

Mike -- Email Ignored wrote:

In std::vector, is reserve or resize required?

No, but you need to fill the vector somehow. You can do that either upon
construction, or by using push_back, insert, or resize.

The reserve() method, on the other hand does not grow the vector.

>
On:
Linux mbrc32 2.6.22.1-41.fc7 #1 SMP Fri
Jul 27 18:10:34 EDT 2007 i686 athlon
i386 GNU/Linux
Using:
g++ (GCC) 4.1.2 20070502 (Red Hat 4.1.2-12)
>
The program below fails, but if the reserve(en)
is uncommented, it works.

It does not. You are observing a manifestation of undefined behavior.

Is this as expected?

There are no expectations as to how undefined behavior will show itself.

From a quality of implementation point of view, would want to see an abort
if you compile the program with assertions turned on.

// vectst.cc 07/04/08
>
#include <iostream>
#include <vector>
using namespace std;
>
int main(int argc, const char* argv[])
{
int en = 10;
vector<int vec;
>
// vec.reserve(en) ;
for (int jj = 0; jj < en; ++jj)
vec[jj] = jj;
for (int jj = 0; jj < en; ++jj)
cout << vec[jj] << endl;

Try something like

cout << vec.size() << endl;

and ponder the meaning of what you get.

>
exit (0);
}

Best

Kai-Uwe Bux

[callumurquhart@googlemail.com]

Re: std::vector: reserve required?


http://www.cplusplus.com/reference/stl/vector/operator[].html

[acehreli@gmail.com]

Re: std::vector: reserve required?

On Jul 4, 7:44 am, Darío Griffo <dario.griffo.l is...@gmail.com wrote:

Mike -- Email Ignored wrote:
>
>
>

In std::vector, is reserve or resize required?

>

On:
   Linux mbrc32 2.6.22.1-41.fc7 #1 SMP Fri
      Jul 27 18:10:34 EDT 2007 i686 athlon
      i386 GNU/Linux
Using:
   g++ (GCC) 4.1.2 20070502 (Red Hat 4.1.2-12)

>

The program below fails, but if the reserve(en)
is uncommented, it works.  Is this as expected?

>

// vectst.cc 07/04/08

>

#include <iostream>
#include <vector>
using namespace std;

>

int main(int argc, const char* argv[])
{
   int                  en = 10;
   vector<int         vec;

>

//   vec.reserve(en) ;
   for (int jj = 0; jj < en; ++jj)
      vec[jj] = jj;
   for (int jj = 0; jj < en; ++jj)
      cout << vec[jj] << endl;

>

   exit (0);
}

>

Thanks,
Mike.

>
No, you could do that (use reserve), or, use push_back()

reserve() does not make the vector larger. You must have meant "you
could use resize or push_back."

Ali

[acehreli@gmail.com]

Re: std::vector: reserve required?

On Jul 4, 7:54 am, callumurquh...@ googlemail.com wrote:

http://www.cplusplus.com/reference/stl/vector/operator[].html

If the OP did not concentrate on the vec.reserve(en) , the document you
show could be used to help with the problem. But because the OP does
expect some behavior from vec.reserve(en) , the problem is not with
operator[].

Ali

[Mike -- Email Ignored]

Re: std::vector: reserve required?

On Fri, 04 Jul 2008 08:30:37 -0700, acehreli wrote:

On Jul 4, 7:54 am, callumurquh...@ googlemail.com wrote:

>http://www.cplusplus.com/reference/stl/vector/operator[].html

>
If the OP did not concentrate on the vec.reserve(en) , the document you
show could be used to help with the problem. But because the OP does
expect some behavior from vec.reserve(en) , the problem is not with
operator[].
>
Ali

Another test using:
vec.reserve(en) ;
for (int jj = 0; jj < en; ++jj)
vec[jj] = jj;
cout << "vec.size() = " << vec.size() << endl;

prevents the crash, but vec.size() returns zero, showing that
in this case, reserve() really does not work.

Mike.

[=?ISO-8859-1?Q?Dar=EDo_Griffo?=]

Re: std::vector: reserve required?


acehr...@gmail. com wrote:

reserve() does not make the vector larger. You must have meant "you
could use resize or push_back."
>
Ali

Yes, you're right

[LR]

Re: std::vector: reserve required?

Mike -- Email Ignored wrote:

On Fri, 04 Jul 2008 08:30:37 -0700, acehreli wrote:
>

>On Jul 4, 7:54 am, callumurquh...@ googlemail.com wrote:

>>http://www.cplusplus.com/reference/stl/vector/operator[].html

>If the OP did not concentrate on the vec.reserve(en) , the document you
>show could be used to help with the problem. But because the OP does
>expect some behavior from vec.reserve(en) , the problem is not with
>operator[].
>>
>Ali

>
Another test using:
vec.reserve(en) ;
for (int jj = 0; jj < en; ++jj)
vec[jj] = jj;
cout << "vec.size() = " << vec.size() << endl;
>
prevents the crash, but vec.size() returns zero,

What does vec.capacity() return?

showing that
in this case, reserve() really does not work.

I think it doesn't do what you expect. But why do you think it doesn't work?

Please consider this:
#include <iostream>
#include <vector>

int main() {
const unsigned int en = 10;
std::vector<int v;
//
std::cout << "a " << v.size() << " " << v.capacity() << std::endl;
v.reserve(en);
std::cout << "b " << v.size() << " " << v.capacity() << std::endl;
//
for(unsigned int i=0; i<en; i++) {
v[i] = i;
}
//
v.push_back(-8);
std::cout << "c " << v.size() << " " << v.capacity() << std::endl;
//
for(unsigned int i=0; i<en; i++) {
std::cout << "[" << i << "] " << v[i] << std::endl;
}
}


Also, please consider this:
#include <iostream>
#include <vector>

int main() {
const unsigned int en = 10;
std::vector<int v;
std::cout << "a " << v.size() << " " << v.capacity() << std::endl;
v.resize(en); // there's more than one way to do it
std::cout << "b " << v.size() << " " << v.capacity() << std::endl;
//
v[0] = -8;
v[9] = -7;
v.push_back(-9);
std::cout << "c " << v.size() << " " << v.capacity() << std::endl;
std::copy(v.beg in(), v.end(), std::ostream_it erator<int>(std ::cout,
" "));
}


LR

[red floyd]

Re: std::vector: reserve required?

Mike -- Email Ignored wrote:

On Fri, 04 Jul 2008 08:30:37 -0700, acehreli wrote:
>

>On Jul 4, 7:54 am, callumurquh...@ googlemail.com wrote:

>>http://www.cplusplus.com/reference/stl/vector/operator[].html

>If the OP did not concentrate on the vec.reserve(en) , the document you
>show could be used to help with the problem. But because the OP does
>expect some behavior from vec.reserve(en) , the problem is not with
>operator[].
>>
>Ali

>
Another test using:
vec.reserve(en) ;
for (int jj = 0; jj < en; ++jj)
vec[jj] = jj;
cout << "vec.size() = " << vec.size() << endl;
>
prevents the crash, but vec.size() returns zero, showing that
in this case, reserve() really does not work.
>

No, reserve() does work. You misunderstand how it does.
vector<>::reser ve changes the CAPACITY -- that is, you can use
push_back() or resize() up to the amount you've reserved without
the vector reallocating. It does not change the SIZE of the vector.

Try this:

#include <iostream>
#include <ostream>
#include <vector>

using namespace std;

int main()
{
vector<intv;

cout << "size = " << v.size() << "\n"
<< "capacity = " << v.capacity << "\n";

v.reserve(200);

cout << "size = " << v.size() << "\n"
<< "capacity = " << v.capacity << "\n";

v.resize(200);

cout << "size = " << v.size() << "\n"
<< "capacity = " << v.capacity << endl;

return 0;
}

[red floyd]

Re: std::vector: reserve required?

red floyd wrote:

Mike -- Email Ignored wrote:

>On Fri, 04 Jul 2008 08:30:37 -0700, acehreli wrote:
>>

>>On Jul 4, 7:54 am, callumurquh...@ googlemail.com wrote:
>>>http://www.cplusplus.com/reference/stl/vector/operator[].html
>>If the OP did not concentrate on the vec.reserve(en) , the document you
>>show could be used to help with the problem. But because the OP does
>>expect some behavior from vec.reserve(en) , the problem is not with
>>operator[].
>>>
>>Ali

>>
>Another test using:
> vec.reserve(en) ;
> for (int jj = 0; jj < en; ++jj)
> vec[jj] = jj;
> cout << "vec.size() = " << vec.size() << endl;
>>
>prevents the crash, but vec.size() returns zero, showing that
>in this case, reserve() really does not work.
>>

>
No, reserve() does work. You misunderstand how it does.
vector<>::reser ve changes the CAPACITY -- that is, you can use
push_back() or resize() up to the amount you've reserved without
the vector reallocating. It does not change the SIZE of the vector.
>
Try this:
>
#include <iostream>
#include <ostream>
#include <vector>
>
using namespace std;
>
int main()
{
vector<intv;
>
cout << "size = " << v.size() << "\n"
<< "capacity = " << v.capacity << "\n";
>
v.reserve(200);
>
cout << "size = " << v.size() << "\n"
<< "capacity = " << v.capacity << "\n";
>
v.resize(200);
>
cout << "size = " << v.size() << "\n"
<< "capacity = " << v.capacity << endl;
>
return 0;
}

Crap. Typo. All those v.capacity calls should be v.capacity()

[James Kanze]

Re: std::vector: reserve required?

On Jul 4, 4:50 pm, Kai-Uwe Bux <jkherci...@gmx .netwrote:

Mike -- Email Ignored wrote:

On:
Linux mbrc32 2.6.22.1-41.fc7 #1 SMP Fri
Jul 27 18:10:34 EDT 2007 i686 athlon
i386 GNU/Linux
Using:
g++ (GCC) 4.1.2 20070502 (Red Hat 4.1.2-12)

The program below fails, but if the reserve(en) is
uncommented, it works.

I get a core dump from g++ 4.2.1, when I compile with the usual
options. With or without the reserve() commented out. You
probably forgot the necessary options to make g++ usable. (One
of these days, someone will come out with a compiler which is
usable without special options. But it's not happened yet.)

It does not. You are observing a manifestation of undefined
behavior.

Is this as expected?

There are no expectations as to how undefined behavior will
show itself.

Yes and no. From experience, I find that undefined behavior
usually works in all of your tests, and then fails in the most
embarassing way possible in the demo before the most important
client.

From a quality of implementation point of view, would want to
see an abort if you compile the program with assertions turned
on.

You do with g++. I'm pretty sure you also do with VC++, but I
don't have a Windows machine handy here to test it with.

--
James Kanze (GABI Software) email:james.kan ze@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientier ter Datenverarbeitu ng
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34

[Mike -- Email Ignored]

Re: std::vector: reserve required?

On Fri, 04 Jul 2008 13:13:43 -0700, James Kanze wrote:

On Jul 4, 4:50 pm, Kai-Uwe Bux <jkherci...@gmx .netwrote:

>Mike -- Email Ignored wrote:

>

On:
Linux mbrc32 2.6.22.1-41.fc7 #1 SMP Fri
Jul 27 18:10:34 EDT 2007 i686 athlon
i386 GNU/Linux
Using:
g++ (GCC) 4.1.2 20070502 (Red Hat 4.1.2-12)

>

The program below fails, but if the reserve(en) is uncommented, it
works.

>
I get a core dump from g++ 4.2.1, when I compile with the usual options.
With or without the reserve() commented out. You probably forgot the
necessary options to make g++ usable. (One of these days, someone will
come out with a compiler which is usable without special options. But
it's not happened yet.)

You may have forgotten some option if you get a failure with reserve()
uncommented; in my case options are correct as verified by extensive
use in other contexts.

>

>It does not. You are observing a manifestation of undefined behavior.

>

Is this as expected?

>

>There are no expectations as to how undefined behavior will show
>itself.

>
Yes and no. From experience, I find that undefined behavior usually
works in all of your tests, and then fails in the most embarassing way
possible in the demo before the most important client.
>

I guess I should have said I was referring to the standard as is
usually the case on this group. There is no "Yes and no".

>From a quality of implementation point of view, would want to see an
>abort if you compile the program with assertions turned on.

>
You do with g++. I'm pretty sure you also do with VC++, but I don't
have a Windows machine handy here to test it with.

I see that in you use vec.at(jj) instead of vec[jj], it throws
an exception if the index is out of range. I changed the code to use
vec.at(jj) where there is uncertainly. My problem is solved.

Mike.

[Mike -- Email Ignored]

Re: std::vector: reserve required?

On Fri, 04 Jul 2008 20:25:40 +0000, Mike -- Email Ignored wrote:

On Fri, 04 Jul 2008 13:13:43 -0700, James Kanze wrote:

[...]

>I get a core dump from g++ 4.2.1, when I compile with the usual
>options.
> With or without the reserve() commented out. You probably forgot the
>necessary options to make g++ usable. (One of these days, someone will
>come out with a compiler which is usable without special options. But
>it's not happened yet.)

>
You may have forgotten some option if you get a failure with reserve()
uncommented; in my case options are correct as verified by extensive use
in other contexts.
>

Correction. You may have no option error. As indicated elsewhere
in this thread, reserve() is never correct in the original code
presented. Hopefully, if you use resize() instead, the code will work.

[...]

Mike.

[James Kanze]

Re: std::vector: reserve required?

On Jul 4, 10:25 pm, Mike -- Email Ignored <m_d_berger_1.. .@yahoo.com>
wrote:

On Fri, 04 Jul 2008 13:13:43 -0700, James Kanze wrote:

On Jul 4, 4:50 pm, Kai-Uwe Bux <jkherci...@gmx .netwrote:

Mike -- Email Ignored wrote:

On:
Linux mbrc32 2.6.22.1-41.fc7 #1 SMP Fri
Jul 27 18:10:34 EDT 2007 i686 athlon
i386 GNU/Linux
Using:
g++ (GCC) 4.1.2 20070502 (Red Hat 4.1.2-12)

The program below fails, but if the reserve(en) is
uncommented, it works.

I get a core dump from g++ 4.2.1, when I compile with the
usual options. With or without the reserve() commented out.
You probably forgot the necessary options to make g++
usable. (One of these days, someone will come out with a
compiler which is usable without special options. But it's
not happened yet.)

You may have forgotten some option if you get a failure with
reserve() uncommented; in my case options are correct as
verified by extensive use in other contexts.

I used the usual options that I use for compiling production
code:
-std=c++98
-ffor-scope
-fno-gnu-keywords
-foperator-names
-pipe
-Wall
-W
-Wno-sign-compare
-Wno-deprecated
-Wno-non-virtual-dtor
-Wpointer-arith
-Wno-unused
-Wno-switch
-Wno-missing-field-initializers
-ggdb3
-D_GLIBCXX_CONCE PT_CHECKS
-D_GLIBCXX_DEBUG
-D_GLIBCXX_DEBUG _PEDANTIC
They're not complete (I forget why we don't have -pedantic in
there), but they're a start.

The important ones for error checking in the library are the
last three. Logically, they should be the default, but hey, no
compiler I know gets the defaults right.

It does not. You are observing a manifestation of undefined
behavior.

Is this as expected?

>

There are no expectations as to how undefined behavior will
show itself.

Yes and no. From experience, I find that undefined behavior
usually works in all of your tests, and then fails in the
most embarassing way possible in the demo before the most
important client.

I guess I should have said I was referring to the standard as is
usually the case on this group. There is no "Yes and no".

I guess I should have explained in detail that this was meant as
a somewhat humorous characterizatio n. It didn't occur to me
that anyone would miss this.

From a quality of implementation point of view, would want
to see an abort if you compile the program with assertions
turned on.

You do with g++. I'm pretty sure you also do with VC++, but
I don't have a Windows machine handy here to test it with.

I see that in you use vec.at(jj) instead of vec[jj], it throws
an exception if the index is out of range. I changed the code
to use vec.at(jj) where there is uncertainly. My problem is
solved.

Except that you want an abort, not an exception. (The problem
isn't cases where there is uncertainly. Uncertainty can be
removed by means of an if. The problem is where your certitudes
turn out to be wrong.)

--
James Kanze (GABI Software) email:james.kan ze@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientier ter Datenverarbeitu ng
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34