C++ cout printing slowly

NOTE: This experimental result is valid for MSVC. In some other implementation of library, the result will vary.

printf could be (much) faster than cout. Although printf parses the format string in runtime, it requires much less function calls and actually needs small number of instruction to do a same job, comparing to cout. Here is a summary of my experimentation:

The number of static instruction

In general, cout generates a lot of code than printf. Say that we have the following cout code to print out with some formats.

os << setw(width) << dec << "0x" << hex << addr << ": " << rtnname <<
  ": " << srccode << "(" << dec << lineno << ")" << endl;

On a VC++ compiler with optimizations, it generates around 188 bytes code. But, when you replace it printf-based code, only 42 bytes are required.

The number of dynamically executed instruction

The number of static instruction just tells the difference of static binary code. What is more important is the actual number of instruction that are dynamically executed in runtime. I also did a simple experimentation:

Test code:

int a = 1999;
char b = 'a';
unsigned int c = 4200000000;
long long int d = 987654321098765;
long long unsigned int e = 1234567890123456789;
float f = 3123.4578f;
double g = 3.141592654;

void Test1()
{
    cout 
        << "a:" << a << “\n”
        << "a:" << setfill('0') << setw(8) << a << “\n”
        << "b:" << b << “\n”
        << "c:" << c << “\n”
        << "d:" << d << “\n”
        << "e:" << e << “\n”
        << "f:" << setprecision(6) << f << “\n”
        << "g:" << setprecision(10) << g << endl;
}

void Test2()
{
    fprintf(stdout,
        "a:%d\n"
        "a:%08d\n"
        "b:%c\n"
        "c:%u\n"
        "d:%I64d\n"
        "e:%I64u\n"
        "f:%.2f\n"
        "g:%.9lf\n",
        a, a, b, c, d, e, f, g);
    fflush(stdout);
}

int main()
{
    DWORD A, B;
    DWORD start = GetTickCount();
    for (int i = 0; i < 10000; ++i)
        Test1();
    A = GetTickCount() - start;

    start = GetTickCount();
    for (int i = 0; i < 10000; ++i)
        Test2();
    B = GetTickCount() - start;
    
    cerr << A << endl;
    cerr << B << endl;
    return 0;
}

Here is the result of Test1 (cout):

• # of executed instruction: 423,234,439
• # of memory loads/stores: approx. 320,000 and 980,000
• Elapsed time: 52 seconds

Then, what about printf? This is the result of Test2:

• # of executed instruction: 164,800,800
• # of memory loads/stores: approx. 70,000 and 180,000
• Elapsed time: 13 seconds

In this machine and compiler, printf was much faster cout. In both number of executed instructions, and # of load/store (indicates # of cache misses) have 3~4 times differences.

I know this is an extreme case. Also, I should note that cout is much easier when you're handling 32/64-bit data and require 32/64-platform independence. There is always trade-off. I'm using cout when checking type is very tricky.

Okay, cout in MSVS just sucks :)

I would suggest you try this same test on a different computer. I don't have a good answer for why this might be happening; all I can say is I have never noticed a speed difference between cout and printf. I also tested your code using gcc 4.3.2 on Linux and there was no difference whatsoever.

That being said, you can't easily replace cout with your own implementation. The fact is, cout is an instance of std::ostream which has a lot of functionality built into it which is necessary for interoperability with other classes that overload the iostream operators.

Edit:

Anyone that says printf is always faster than std::cout is simply wrong. I just ran the test code posted by minjang, with gcc 4.3.2 and the -O2 flag on a 64-bit AMD Athlon X2, and cout was actually faster.

I got the following results:

printf: 00:00:12.024
cout:   00:00:04.144

Is cout always faster than printf? Probably not. Especially not with older implementations. But on newer implementations iostreams are likely to be faster than stdio because instead of parsing a format string at runtime, the compiler knows at compile time what functions it needs to call in order to convert integers/floats/objects to strings.

But more importantly, the speed of printf versus cout depends on the implementation, and so the problem described by the OP is not easily explicable.

Try call ios::sync_with_stdio(false); before using std::cout/cin, unless of course, you mix stdio and iostream in your program, which is a bad thing to do.

Based on my experience in programming competitions, printf IS faster than cout.

I remember many times when my solution didn't make it before the Time limit just because of cin/cout, while printf/scanf did work.

Besides that, it seems normal (at least for me) that cout is slower than printf, because it does more operations.