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

A

Alex Vinokur

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?
 
U

usr.root

Alex Vinokur 写é“:
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!
 
M

Mike Wahler

Alex Vinokur said:
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
 
A

Alex Vinokur

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
 
A

Alex Vinokur

Mike Wahler said:
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
Exactly the number you used: 100
Then undefined behavior happens.


This is an error. Of course, my mistake.
At least one argument (the new size)
is required.


Exactly the number you used: 200
See my new message at
http://groups.google.com/group/comp.lang.c++/msg/04ad33960f339af5
 
P

persenaama

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().
 
M

msalters

(e-mail address removed) schreef:
Alex Vinokur 写é“:


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
 
A

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.
 

Ask a Question

Want to reply to this thread or ask your own question?

You'll need to choose a username for the site, which only take a couple of moments. After that, you can post your question and our members will help you out.

Ask a Question

Members online

Forum statistics

Threads
473,744
Messages
2,569,482
Members
44,901
Latest member
Noble71S45

Latest Threads

Top