Question

After performing some tests I noticed that printf is much faster than cout. I know that it’s implementation dependent, but on my Linux box printf is 8x faster. So my idea is to mix the two printing methods: I want to use cout for simple prints, and I plan to use printf for producing huge outputs (typically in a loop). I think it’s safe to do as long as I don’t forget to flush before switching to the other method:

cout << "Hello" << endl;
cout.flush();

for (int i=0; i<1000000; ++i) {
    printf("World!\n");
}
fflush(stdout);

cout << "last line" << endl;
cout << flush;

Is it OK like that?

Update: Thanks for all the precious feedbacks. Summary of the answers: if you want to avoid tricky solutions, simply stick with cout but don’t use endl since it flushes the buffer implicitly (slowing the process down). Use "\n" instead. It can be interesting if you produce large outputs.

Answer 1

The direct answer is that yes, that’s okay.

A lot of people have thrown around various ideas of how to improve speed, but there seems to be quite a bit of disagreement over which is most effective. I decided to write a quick test program to get at least some idea of which techniques did what.

#include <iostream>
#include <string>
#include <sstream>
#include <time.h>
#include <iomanip>
#include <algorithm>
#include <iterator>
#include <stdio.h>

char fmt[] = "%s\n";
static const int count = 3000000;
static char const *const string = "This is a string.";
static std::string s = std::string(string) + "\n";

void show_time(void (*f)(), char const *caption) { 
    clock_t start = clock();
    f();
    clock_t ticks = clock()-start;
    std::cerr << std::setw(30) << caption 
        << ": " 
        << (double)ticks/CLOCKS_PER_SEC << "\n";
}

void use_printf() {
    for (int i=0; i<count; i++)
        printf(fmt, string);
}

void use_puts() {
    for (int i=0; i<count; i++) 
        puts(string);        
}

void use_cout() { 
    for (int i=0; i<count; i++)
        std::cout << string << "\n";
}

void use_cout_unsync() { 
    std::cout.sync_with_stdio(false);
    for (int i=0; i<count; i++)
        std::cout << string << "\n";
    std::cout.sync_with_stdio(true);
}

void use_stringstream() { 
    std::stringstream temp;
    for (int i=0; i<count; i++)
        temp << string << "\n";
    std::cout << temp.str();
}

void use_endl() { 
    for (int i=0; i<count; i++)
        std::cout << string << std::endl;
}

void use_fill_n() { 
    std::fill_n(std::ostream_iterator<char const *>(std::cout, "\n"), count, string);
}

void use_write() {
    for (int i = 0; i < count; i++)
        std::cout.write(s.data(), s.size());
}

int main() { 
    show_time(use_printf, "Time using printf");
    show_time(use_puts, "Time using puts");
    show_time(use_cout, "Time using cout (synced)");
    show_time(use_cout_unsync, "Time using cout (un-synced)");
    show_time(use_stringstream, "Time using stringstream");
    show_time(use_endl, "Time using endl");
    show_time(use_fill_n, "Time using fill_n");
    show_time(use_write, "Time using write");
    return 0;
}

I ran this on Windows after compiling with VC++ 2013 (both x86 and x64 versions). Output from one run (with output redirected to a disk file) looked like this:

          Time using printf: 0.953
            Time using puts: 0.567
   Time using cout (synced): 0.736
Time using cout (un-synced): 0.714
    Time using stringstream: 0.725
            Time using endl: 20.097
          Time using fill_n: 0.749
           Time using write: 0.499

As expected, results vary, but there are a few points I found interesting:

1. printf/puts are much faster than cout when writing to the NUL device
• but cout keeps up quite nicely when writing to a real file
2. Quite a few proposed optimizations accomplish little
   • In my testing, fill_n is about as fast as anything else
3. By far the biggest optimization is avoiding endl
4. cout.write gave the fastest time (though probably not by a significant margin

I’ve recently edited the code to force a call to printf. Anders Kaseorg was kind enough to point out–that g++ recognizes the specific sequence printf("%s\n", foo); is equivalent to puts(foo);, and generates code accordingly (i.e., generates code to call puts instead of printf). Moving the format string to a global array, and passing that as the format string produces identical output, but forces it to be produced via printf instead of puts. Of course, it’s possible they might optimize around this some day as well, but at least for now (g++ 5.1) a test with g++ -O3 -S confirms that it’s actually calling printf (where the previous code compiled to a call to puts).