Hi,
As I read in the archives, the performance problem caused by memory
reallocations during vector::push_ba ck is a common one. My first C++
program is suffering from it: 300 thousand push_backs result,
according to the profiler, in 20 reallocations; these 20 reallocations
account for 3.6 seconds (Celeron 1.3G), which is 40% of total
execution time.
What I don't understand: why is the reallocation code so complex? I
studied the library source and I have a hard time understanding it,
but it seems to be copying the vector item by item in each
reallocation. Why wouldn't a "realloc" suffice?
And, given that I don't know the vector size beforehand, is there
anything else I can do other than trying deqeue or a guessed
vector::reserve ?
In case it matters, I'm using gcc 3.3 with its standard c++ library on
a Debian sarge, but portability is also an issue.
Thanks!
Jul 22 '05
30  4508 
"David Harmon" <so****@netcom. com> wrote in message
news:41******** *******@news.we st.earthlink.ne t... On Thu, 30 Sep 2004 14:03:10 GMT in comp.lang.c++, "Andrew Koenig"
<ar*@acm.org> wrote,
If I insert the statement
std::cout << v.size() << std::endl;
in the appropriate place, it prints 1000000 and still runs in 0.03
seconds.
But perhaps the compiler is determining statically what the size of the
vector would be at that point and encoding only that string in the
executable. I think you have to read the initial 1000000 from a file
that is unknown at compile time.
Possible but unlikely.
How about trying whatever tests you feel are appropriate on your own machine
and reporting the results? This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics | |
by: Hitesh Bhatiya |
last post by:
Hi all,
I have written a small program to accept some socket connections, which are
then added to a vector (using push_back). But after a few calls to the
push_back function, it deleted the object that was added last.
Could someone please tell me why this happens ? Am I doing something wrong
here ?
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by: Mark P |
last post by:
Say I have objects of class C which are fairly large. Then consider:
vector<C> vc;
vc.push_back(C());
Naively this would seem to construct a temporary object C(), copy it
into the space owned by the vector, and then delete the temporary object.
I realize this is implementation dependent, but will most modern
compilers optimize away the creation of the temporary object, or is
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by: whocares |
last post by:
hi everyone.
i'm currently experiencing a strange problem under vc++ 2005 express. i hope
someone has a hint for me, i'm kind of lost atm.
i'm using a vectors of pointers in my code.
using the release build i can add and remove elements aslong as i stay below
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by: Al |
last post by:
What is the problem with this code? It always crashes at the
'push_back'. I found that thanks to debugging. Any Ideas?
Other question: what is a faster way to convert a string to an integer?
Is there a built-in function to do that?
Here's the code:
#include <iostream>
#include <fstream>
#include <vector>
#include <string>
|
by: ma740988 |
last post by:
Consider the source snippet
int main()
{
std::vector<double> vec_push( 0x10 );
size_t const SZ = vec_push.size();
std::cout << vec_push.size() << std::endl;
if ( SZ != 0x10 ) {
cout << " something's amiss " << endl;
| | |
by: Siam |
last post by:
Hi,
I'm a little new to stl so bear with me...Say I have the following
code:
vector<intvec;
int i = 3;
vec.push_back(i);
i=4;
cout<<vec.at(0)<<endl;
|
by: jmsanchezdiaz |
last post by:
CPP question: if i had a struct like "struct str { int a; int b };"
and a vector "std::vector < str test;" and wanted to push_back a
struct, would i have to define the struct, fill it, and then push_back
it, or could i pushback the two ints directly somehow?
Thanks for all.
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