Is std::vector so much slower than plain arrays?

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南方客 2020-11-22 12:00

I\'ve always thought it\'s the general wisdom that std::vector is \"implemented as an array,\" blah blah blah. Today I went down and tested it, and it seems to

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不要未来只要你来 (楼主)

2020-11-22 12:20

By the way the slow down your seeing in classes using vector also occurs with standard types like int. Heres a multithreaded code:

#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
using namespace std;

//pthread_mutex_t map_mutex=PTHREAD_MUTEX_INITIALIZER;

long long num=500000000;
int procs=1;

struct iterate
{
    int id;
    int num;
    void * member;
    iterate(int a, int b, void *c) : id(a), num(b), member(c) {}
};

//fill out viterate and piterate
void * viterate(void * input)
{
    printf("am in viterate\n");
    iterate * info=static_cast (input);
    // reproduce member type
    vector test= *static_cast*> (info->member);
    for (int i=info->id; inum)
    {
        //printf("am in viterate loop\n");
        test[i];
    }
    pthread_exit(NULL);
}

void * piterate(void * input)
{
    printf("am in piterate\n");
    iterate * info=static_cast (input);;
    int * test=static_cast (info->member);
    for (int i=info->id; inum) {
        //printf("am in piterate loop\n");
        test[i];
    }
    pthread_exit(NULL);
}

int main()
{
    cout<<"producing vector of size "< vtest(num);
    cout<<"produced  a vector of size "<member=&pint;
        ans=pthread_create(&thread[i], NULL, piterate, (void*) it[i]);
    }
    for (int i=0; i



The behavior from the code shows the instantiation of vector is the longest part of the code. Once you get through that bottle neck. The rest of the code runs extremely fast. This is true no matter how many threads you are running on. 

By the way ignore the absolutely insane number of includes. I have been using this code to test things for a project so the number of includes keep growing.