std::vector's reserve(), erase() and clear()
Alex Vinokur
vector<int> v;
v.reserve(100);
v.resize(100);
v.erase(v.end());
How many elements are reserved here: 100 or 99?
v.reserve(200);
v.resize();
v.clear();
How many elements are reserved here: 200 or 0?
--
Alex Vinokur
email: alex DOT vinokur AT gmail DOT com
http://mathforum.org/library/view/10978.html
http://sourceforge.net/users/alexvn
usr....@gmail.com
Alex Vinokur 写道：
you can test it on you computer,it's easy!
Mike Wahler
"Alex Vinokur" <ale...@go.to> wrote in message
news:dgvvuj$mh2$1...@reader.greatnowhere.com...
> What is relation between std::vector's reserve() and erase()/clear()?
>
> vector<int> v;
>
> v.reserve(100);
> v.resize(100);
> v.erase(v.end());
This last line will produce undefined behavior. 'end()'
points to one element past the last element in
the vector. You can't 'erase' it.
>
> How many elements are reserved here: 100 or 99?
Exactly the number you used: 100
Then undefined behavior happens.
>
> v.reserve(200);
> v.resize();
This is an error. At least one argument (the new size)
is required.
> v.clear();
>
> How many elements are reserved here: 200 or 0?
Exactly the number you used: 200
But the call to resize is in error.
-Mike
Alex Vinokur
<usr....@gmail.com> wrote in message news:1127453368.6...@g14g2000cwa.googlegroups.com...
Alex Vinokur ??:
> What is relation between std::vector's reserve() and erase()/clear()?
[snip]
you can test it on you computer,it's easy!
====== foo.cpp ======
#include <vector>
#include <iostream>
using namespace std;
#define SHOW cout << "capacity = " << v.capacity() << "\t size = " << v.size() << endl;
int main()
{
vector<int> v;
SHOW;
v.reserve(100);
SHOW;
v.resize(100);
SHOW;
v.erase(v.end() - 1);
SHOW;
v.clear();
SHOW;
return 0;
}
=====================
1. Output for GNU g++ 3.4.4, GNU gpp 4.0.1, Bolrand C++ 5.5.1, Digital Mars 8.38n
---------------------------
capacity = 0 size = 0
capacity = 100 size = 0
capacity = 100 size = 100
capacity = 100 size = 99
capacity = 100 size = 0
---------------------------
2. Output for Microsoft C++ 13.00.9466
---------------------------
capacity = 0 size = 0
capacity = 100 size = 0
capacity = 100 size = 100
capacity = 100 size = 99
capacity = 0 size = 0
---------------------------
We can see that capacity == 0 after clear() for Microsoft C++.
Is it correct behavior?
Alex Vinokur
"Mike Wahler" <mkwa...@mkwahler.net> wrote in message news:APMYe.2466$0m6....@newsread3.news.pas.earthlink.net...[snip]
> > v.erase(v.end());
>
> This last line will produce undefined behavior. 'end()'
> points to one element past the last element in
> the vector. You can't 'erase' it.
Thanks. Updated in my new message at
http://groups.google.com/group/comp.lang.c++/msg/04ad33960f339af5
>
> >
> > How many elements are reserved here: 100 or 99?
>
> Exactly the number you used: 100
> Then undefined behavior happens.
>
> >
> > v.reserve(200);
> > v.resize();
>
> This is an error.
Of course, my mistake.
> At least one argument (the new size)
> is required.
>
> > v.clear();
> >
> > How many elements are reserved here: 200 or 0?
>
> Exactly the number you used: 200
See my new message at
http://groups.google.com/group/comp.lang.c++/msg/04ad33960f339af5
> But the call to resize is in error.
>
> -Mike
>
>
--
persenaama
v.capacity() could tell you if you fix the erase().
> How many elements are reserved here: 200 or 0?
v.capacity() could tell you if you fix the resize().
msalters
usr....@gmail.com schreef:
> Alex Vinokur 写道：
>
> > What is relation between std::vector's reserve() and erase()/clear()?
>
> you can test it on you computer,it's easy!
Not really. I.e. If I call reserve(100), and get capacity()==128,
that's OK. However, you might get capacity()==100 and that's OK too.
You can't induce the C++ standard.
HTH,
Michiel Salters
Andrew Koenig
> What is relation between std::vector's reserve() and erase()/clear()?
v.reserve(n) sets the capacity to at least max(n, v.size()) and leaves the
size unchanged.
v.erase reduces the size by the number of elements erased and does not
change the capacity.
v.clear sets the size to 0 and does not change the capacity.
> vector<int> v;
v's size is now 0.
> v.reserve(100);
v's size is now 0 and its capacity is >= 100.
> v.resize(100);
v's size is now 100 and its capacity is >= 100.
> v.erase(v.end());
This is undefined behavior because v.end() does not refer to an element.
> How many elements are reserved here: 100 or 99?
> v.reserve(200);
v's size is now 100 and its capacity is >= 200.
> v.resize();
This should not compile, as you did not specify a size.
> v.clear();
v's size is now 0 and its capacity is >= 200.
Alex Vinokur
"Andrew Koenig" <a...@acm.org> wrote in message news:VvUYe.300725$5N3.2...@bgtnsc05-news.ops.worldnet.att.net...
[snip]
> v.clear sets the size to 0 and does not change the capacity.
The issue was discussed in microsoft.public.vc.language as well.
See Tom Widmer [VC++ MVP]'s message at
http://groups.google.com/group/microsoft.public.vc.language/msg/c3297d544d6a8a18