为什么 cin/cout 比 scanf/ printf 慢

Question

#include <iostream>
using namespace std;

int main(){
    int t;
    long long int n,res,x;
    scanf("%d",&t);
    while(t--){
            scanf("%lld",&n);
            res=0;


    for(int i=0;i<n;i++){
            scanf("%lld",&x);
            res^=x;
    }
    if(res==0)
        printf("-1\n");
    else 
        printf("%lld\n",res);

}
return 0;
}

this same program when i used cin and cout was timed out in hackerearth. But passed with scanf and printf.

Answer 1

The speed difference is largely due to the iostream I/O functions maintaining synchronization with the CI/O functions. We can turn this off with a call to std::ios::sync_with_stdio(false);

By default standard C++ streams are synchronized to the standard C stream after each input/output operation.

Once synchronization is turned off, the C++ standard streams are allowed to buffer their I/O independently, you can try and see the time taken will be almost similar (possibly lesser than scanf)