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 8  10645 
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!
"Alex Vinokur" <al****@go.to > wrote in message
news:dg******** **@reader.great nowhere.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
<us******@gmail .com> wrote in message news:11******** **************@ g14g2000cwa.goo glegroups.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
"Mike Wahler" <mk******@mkwah ler.net> wrote in message news:AP******** *********@newsr ead3.news.pas.e arthlink.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
> 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(). 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
"Alex Vinokur" <al****@go.to > wrote in message
news:dg******** **@reader.great nowhere.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. This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics | |
by: Stefan Hhne |
last post by:
Hi,
as I recon, std::vector::clear()'s semantics changed from MS VC++ 6.0 to
MS' DOT.NET - compiler.
In the 6.0 version the capacity() of the vector did not change with the
call to clear(), in DOT.NET the capacity() is reduced to 0.
|
by: enzo |
last post by:
hi all,
i don't understand what's wrong:
1)
std::vector<double> p(10);
doesn't compile:
|
by: Steve |
last post by:
Hi,
I'm using a std::vector to store a list of user defined objects. The vector
may have well over 1000 elements, and I'm suffering a performance hit. If I
use push_back I get a much worse perfomance than if I first define the
vector of a given size, then write to the elements with myvec =
However, I'm currently thinking that it isn't feasible to obtain the vector
size, so really need to resize the vector dynamically as I go. Is...
|
by: Jason Heyes |
last post by:
What is a good way of removing elements from std::vector so that the
elements removed satisfy a predicate and end up stored in another
std::vector. It seems as though the algorithm std::remove_if only achieves
half the job. Here is how I would use std::remove_if to remove elements from
std::vector based on predicate:
v.erase(std::remove_if(v.begin(), v.end(), pred), v.end());
After that line is executed I cannot get back the elements...
|
by: Ross A. Finlayson |
last post by:
I'm trying to write some C code, but I want to use C++'s std::vector.
Indeed, if the code is compiled as C++, I want the container to
actually be std::vector, in this case of a collection of value types or
std::vector<int>.
So where I would use an int* and reallocate it from time to time in C,
and randomly access it via , then I figure to copy the capacity and
reserve methods, because I just need a growable array.
I get to considering...
| | |
by: Jason Heyes |
last post by:
Does the STL have a function like this one?
template <typename T>
void remove(std::vector<T> &v, std::vector<T>::size_type index)
{
std::swap(v, v.back());
v.resize(index);
}
Unlike std::vector::erase, it calls T::operator= only three times no matter
|
by: Dilip |
last post by:
If you reserve a certain amount of memory for a std::vector, what
happens when a reallocation is necessary because I overshot the limit?
I mean, say I reserve for 500 elements, the insertion of 501st element
is going to cause some more allocation -- for arguments sake if the
vector re-grows to accomodate 1000 elements, I play around with it and
completely erase everything in it after I am done. Now how much does
the vector hold? Do I...
|
by: atreya |
last post by:
Hi,
I'm trying to figure out if there are any differences between the
implementation of resize(0) and clear() that a programmer using
std::vector should be aware of!
Could there be any advantage/disadvantage of using one over the other?
I checked the VS7.1 implementation, in which I couldn't find any
difference. Both of them are effectively calling erase(begin(), end()).
|
by: Mike -- Email Ignored |
last post by:
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)
|
by: marktang |
last post by:
ONU (Optical Network Unit) is one of the key components for providing high-speed Internet services. Its primary function is to act as an endpoint device located at the user's premises. However, people are often confused as to whether an ONU can Work As a Router. In this blog post, well explore What is ONU, What Is Router, ONU & Routers main usage, and What is the difference between ONU and Router. Lets take a closer look !
Part I. Meaning of...
|
by: Hystou |
last post by:
Most computers default to English, but sometimes we require a different language, especially when relocating. Forgot to request a specific language before your computer shipped? No problem! You can effortlessly switch the default language on Windows 10 without reinstalling. I'll walk you through it.
First, let's disable language synchronization. With a Microsoft account, language settings sync across devices. To prevent any complications,...
| | |
by: Oralloy |
last post by:
Hello folks,
I am unable to find appropriate documentation on the type promotion of bit-fields when using the generalised comparison operator "<=>".
The problem is that using the GNU compilers, it seems that the internal comparison operator "<=>" tries to promote arguments from unsigned to signed.
This is as boiled down as I can make it.
Here is my compilation command:
g++-12 -std=c++20 -Wnarrowing bit_field.cpp
Here is the code in...
|
by: isladogs |
last post by:
The next Access Europe User Group meeting will be on Wednesday 1 May 2024 starting at 18:00 UK time (6PM UTC+1) and finishing by 19:30 (7.30PM).
In this session, we are pleased to welcome a new presenter, Adolph Dupr who will be discussing some powerful techniques for using class modules.
He will explain when you may want to use classes instead of User Defined Types (UDT). For example, to manage the data in unbound forms.
Adolph will...
|
by: conductexam |
last post by:
I have .net C# application in which I am extracting data from word file and save it in database particularly. To store word all data as it is I am converting the whole word file firstly in HTML and then checking html paragraph one by one.
At the time of converting from word file to html my equations which are in the word document file was convert into image.
Globals.ThisAddIn.Application.ActiveDocument.Select();...
|
by: TSSRALBI |
last post by:
Hello
I'm a network technician in training and I need your help.
I am currently learning how to create and manage the different types of VPNs and I have a question about LAN-to-LAN VPNs.
The last exercise I practiced was to create a LAN-to-LAN VPN between two Pfsense firewalls, by using IPSEC protocols.
I succeeded, with both firewalls in the same network. But I'm wondering if it's possible to do the same thing, with 2 Pfsense firewalls...
|
by: 6302768590 |
last post by:
Hai team
i want code for transfer the data from one system to another through IP address by using C# our system has to for every 5mins then we have to update the data what the data is updated we have to send another system
|
by: muto222 |
last post by:
How can i add a mobile payment intergratation into php mysql website.
| | |
by: bsmnconsultancy |
last post by:
In today's digital era, a well-designed website is crucial for businesses looking to succeed. Whether you're a small business owner or a large corporation in Toronto, having a strong online presence can significantly impact your brand's success. BSMN Consultancy, a leader in Website Development in Toronto offers valuable insights into creating effective websites that not only look great but also perform exceptionally well. In this comprehensive...
| | |
