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std::vector's reserve(), erase() and clear()

What is relation between std::vector's reserve() and erase()/clear()?

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

Sep 23 '05 #1
8 10248

Alex Vinokur 写道:
What is relation between std::vector's reserve() and erase()/clear()?

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


you can test it on you computer,it's easy!

Sep 23 '05 #2

"Alex Vinokur" <al****@go.to> wrote in message
news:dg**********@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
Sep 23 '05 #3

<us******@gmail.com> wrote in message news:11**********************@g14g2000cwa.googlegr oups.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
email: alex DOT vinokur AT gmail DOT com
http://mathforum.org/library/view/10978.html
http://sourceforge.net/users/alexvn


Sep 23 '05 #4

"Mike Wahler" <mk******@mkwahler.net> wrote in message news:AP*****************@newsread3.news.pas.earthl ink.net...

"Alex Vinokur" <al****@go.to> wrote in message [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....ad33960f339af5

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....ad33960f339af5 But the call to resize is in error.

-Mike


--
Alex Vinokur
email: alex DOT vinokur AT gmail DOT com
http://mathforum.org/library/view/10978.html
http://sourceforge.net/users/alexvn

Sep 23 '05 #5
> How many elements are reserved here: 100 or 99?

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().

Sep 23 '05 #6

us******@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

Sep 23 '05 #7
"Alex Vinokur" <al****@go.to> wrote in message
news:dg**********@reader.greatnowhere.com...
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.
Sep 23 '05 #8

"Andrew Koenig" <ar*@acm.org> wrote in message news:Vv*********************@bgtnsc05-news.ops.worldnet.att.net...
[snip]
v.clear sets the size to 0 and does not change the capacity.

[snip]

The issue was discussed in microsoft.public.vc.language as well.
See Tom Widmer [VC++ MVP]'s message at
http://groups.google.com/group/micro...297d544d6a8a18
--
Alex Vinokur
email: alex DOT vinokur AT gmail DOT com
http://mathforum.org/library/view/10978.html
http://sourceforge.net/users/alexvn

Sep 23 '05 #9

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