Answer by Thomas for 'printf' vs. 'cout' in C++

People often claim that printf is much faster. This is largely a myth. I just tested it, with the following results:

cout with only endl                     1461.310252 mscout with only '\n'                      343.080217 msprintf with only '\n'                     90.295948 mscout with string constant and endl      1892.975381 mscout with string constant and '\n'       416.123446 msprintf with string constant and '\n'     472.073070 mscout with some stuff and endl           3496.489748 mscout with some stuff and '\n'           2638.272046 msprintf with some stuff and '\n'         2520.318314 ms

Conclusion: if you want only newlines, use printf; otherwise, cout is almost as fast, or even faster. More details can be found on my blog.

To be clear, I'm not trying to say that iostreams are always better than printf; I'm just trying to say that you should make an informed decision based on real data, not a wild guess based on some common, misleading assumption.

Update: Here's the full code I used for testing. Compiled with g++ without any additional options (apart from -lrt for the timing).

#include <stdio.h>#include <iostream>#include <ctime>class TimedSection {    char const *d_name;    timespec d_start;    public:        TimedSection(char const *name) :            d_name(name)        {            clock_gettime(CLOCK_REALTIME, &d_start);        }        ~TimedSection() {            timespec end;            clock_gettime(CLOCK_REALTIME, &end);            double duration = 1e3 * (end.tv_sec - d_start.tv_sec) +                              1e-6 * (end.tv_nsec - d_start.tv_nsec);            std::cerr << d_name << '\t'<< std::fixed << duration << " ms\n";         }};int main() {    const int iters = 10000000;    char const *text = "01234567890123456789";    {        TimedSection s("cout with only endl");        for (int i = 0; i < iters; ++i)            std::cout << std::endl;    }    {        TimedSection s("cout with only '\\n'");        for (int i = 0; i < iters; ++i)            std::cout << '\n';    }    {        TimedSection s("printf with only '\\n'");        for (int i = 0; i < iters; ++i)            printf("\n");    }    {        TimedSection s("cout with string constant and endl");        for (int i = 0; i < iters; ++i)            std::cout << "01234567890123456789"<< std::endl;    }    {        TimedSection s("cout with string constant and '\\n'");        for (int i = 0; i < iters; ++i)            std::cout << "01234567890123456789\n";    }    {        TimedSection s("printf with string constant and '\\n'");        for (int i = 0; i < iters; ++i)            printf("01234567890123456789\n");    }    {        TimedSection s("cout with some stuff and endl");        for (int i = 0; i < iters; ++i)            std::cout << text << "01234567890123456789"<< i << std::endl;    }    {        TimedSection s("cout with some stuff and '\\n'");        for (int i = 0; i < iters; ++i)            std::cout << text << "01234567890123456789"<< i << '\n';    }    {        TimedSection s("printf with some stuff and '\\n'");        for (int i = 0; i < iters; ++i)            printf("%s01234567890123456789%i\n", text, i);    }}