Re: [Boost-users] Efficiacy of std::vector

Hi, I profiled some example code (not yours) using AQTime. Here is a screen shot of the results. You can see that in the inner loop I am accessing the elements using the [] operator, the array and the vector operations perform almost identically, accessing with at() is quite a bit more expensive. Stefan Janos Vegh wrote:

Hi,

I was interested in the efficiacy of std::vector. Using the test program below, I receive the following results. The "simple" handling is equivalent with the C array access. The [] and iterator access time of the vector element is an order of magnitude worse, than that of the simple access. This is not because of index limit control, it is done by at() and its use increases that access time by a factor of three. What is going on in the background here, that the implementation is so ineffective? ( I did my tests with WinXP/cygwin/g++/time, using different setting of the #define-s at the beginning) Shall I expect similar ineffectivity with the other STL objects?

Regards

Janos

Empty cycle real 0m2.173s user 0m1.892s sys 0m0.080s

STL=1, SIMPLE=1, size=0 real 0m4.917s user 0m4.646s sys 0m0.070s

STL=1, size=0 real 0m33.939s user 0m33.658s sys 0m0.090s

STL=1, ITERATOR=1, size=1000 real 0m34.059s user 0m33.688s sys 0m0.100s

STL=1, AT=1, size=1000 real 1m22.769s user 1m22.368s sys 0m0.090s

#define STL 1 #define SIMPLE 0 #define ITERATOR 1 #define AT 0 #define EMPTY 1 #if STL #include <vector> #endif using namespace std; #include <iostream> #define ARR_SIZE 1000 #define CYCLES 1000000 main (int argc, char** argv) { long int i,j,*P; #if EMPTY cout << "Empty cycle"; #else #if STL #if ITERATOR || AT std::vector<long int>::iterator pos; std::vector<long int> arr(ARR_SIZE); #else std::vector <long int> arr; arr.reserve(ARR_SIZE); #endif

cout << "STL=1"; #if SIMPLE cout << ", SIMPLE=1"; #endif #if ITERATOR cout << ", ITERATOR=1"; #endif #if AT cout << ", AT=1"; #endif cout << ", size=" << arr.size(); #else long int *arr; arr = new long int[ARR_SIZE]; cout << "STL=0"; #endif #endif cout << endl; #if SIMPLE && STL P = &arr[0]; #endif for(j=0;j

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