Fast C++ String Output

If you are writing to stdout, you might not be able to influence this all.

Otherwise, set buffering

• setvbuf http://en.cppreference.com/w/cpp/io/c/setvbuf
• std::nounitbuf http://en.cppreference.com/w/cpp/io/manip/unitbuf
• and untie the input output streams (C++) http://en.cppreference.com/w/cpp/io/basic_ios/tie
• std::ios_base::sync_with_stdio(false) (thanks @Dietmar)

Now, Boost Karma is known to be pretty performant. However, I'd need to know more about your input data.

Meanwhile, try to buffer your writes manually: Live on Coliru

#include <stdio.h>

int getData(int i) { return i; }

int main()
{
    char buf[100*24]; // or some other nice, large enough size
    char* const last = buf+sizeof(buf);
    char* out = buf;

    for (int i = 0; i < 100; i++) {
        out += snprintf(out, last-out, "data: %d\n", getData(i));
    }

    *out = '\0';
    printf("%s", buf);
}

Wow, I can't believe I didn't do this earlier.

const int size = 100;
char data[size];
for (int i = 0; i < size; i++) {
    *(data + i) = getData(i);
}

for (int i = 0; i < size; i++) {
    printf("data: %d\n",*(data + i));
}

As I said, printf was the bottleneck, and sprintf wasn't much of an improvement either. So I decided to avoid any sort of printing until the very end, and use pointers instead