fscanf vs ifstream speed

Question

I conjectured that ifstream would be faster than fscanf because fscanf has to parse the format string every time it runs, whereas, with ifstream, we know at compile time what kind of "thing" we want to read.

But when I ran this quick and dirty benchmark

#include <ctime>
#include <cstdio>
#include <iostream>
#include <fstream>
using namespace std;

#define NUMBER_OF_NUMBERS 100000000

int nums[NUMBER_OF_NUMBERS];

int main(int argc, char** argv) {

    FILE * fp = fopen("nums.txt","r");
    auto start = clock();
    for (int i = 0; i < NUMBER_OF_NUMBERS; i++)
        fscanf(fp,"%d",nums+i);
    auto end = clock();
    fclose(fp);

    auto first = end - start;

    ifstream fin("nums.txt");
    start = clock();
    for (int i = 0; i < NUMBER_OF_NUMBERS; i++)
        fin >> nums[i];
    end = clock();
    fin.close();

    auto second = end - start;

    cout << "CLOCKS_PER_SEC : " << CLOCKS_PER_SEC << endl;
    cout << "first          : " << first << endl;
    cout << "first (sec)    : " << first / CLOCKS_PER_SEC << " seconds" << endl;
    cout << "second         : " << second << endl;
    cout << "second (sec)   : " << second / CLOCKS_PER_SEC << " seconds" << endl;
    cout << "diff           : " << second - first << endl;
    cout << "diff (sec)     : " << (second - first) / CLOCKS_PER_SEC << " seconds" << endl;

    return 0;
}

I got as output the following:

CLOCKS_PER_SEC : 1000000
first          : 12336249
first (sec)    : 12 seconds
second         : 25738587
second (sec)   : 25 seconds
diff           : 13402338
diff (sec)     : 13 seconds

ifstream is more than twice as slow as fscanf. Where does fscanf get all this speed?

EDIT:

I'm on a reasonably modern 64-bit intel mac, using command line tools that come with xcode, in case it is relevant at all.

Answer 1

fscanf does not need to parse the format string at all. It uses a greedy algorithm that looks for the '%' character and then uses a simple switch statement to generate the input. ifstream on the other hand, needs to perform lookups on its vtable to determine how each minute detail of the input is used.

All this being said, fscanf cannot be extended without altering the C library, while ifstream is as easy as sub-classing it.

EDIT: All of this is also part of the fact that C's fscanf library routine has had far more time/work put into it and more opportunities for optimization. The C++ library is only arbitrary consequence of the C++ standard and thus has not had the same scrutiny applied to it.