C++ cout printing slowly

Question

I noticed if I print out a long string(char*) using cout it seems to print 1 character at a time to the screen in Windows 7, Vista, and Linux(using putty) using Visual C++ 2

Answer 1

You should try to write all your data to an ostringstream first, and then use cout on the ostringstream's str(). I am on 64-bit Windows 7 and Test1 was already significantly faster than Test2 (your mileage may vary). Using an ostringstream to build a single string first and then using cout on that further decreased Test1's execution time by a factor of about 3 to 4. Be sure to #include .

I.e., replace

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;
}

with:

void Test1()
{
    ostringstream oss;
    oss
        << "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;
    cout << oss.str();
}

I suspect ostringstream makes this so much faster as a result of not trying to write to the screen each time you call operator<< on cout. I've also noticed through experience that reducing the number of times you write to the screen (by writing more at once) increases performance (again, your mileage may vary).

E.g.,

void Foo1()
{
    for(int i = 0; i < 10000; ++i) {
        cout << "Foo1\n";
    }
}

void Foo2()
{
    std::string s;
    for(int i = 0; i < 10000; ++i) {
        s += "Foo2\n";
    }
    cout << s;
}

void Foo3()
{
    std::ostringstream oss;
    for(int i = 0; i < 10000; ++i) {
        oss << "Foo3\n";
    }
    cout << oss.str();
}

In my case, Foo1 took 1,092ms, Foo2 took 234ms, and Foo3 took 218ms. ostingstreams are your friend. Obviously Foo2 and Foo3 require (trivially) more memory. To compare this against a C-style function, try sprintf into a buffer and then write that buffer using fprintf and you should see still more efficiency over Test2 (though for me this only improved performance of Test2 by about 10% or so; cout and printf are indeed different beasts under the hood).

Compiler: MinGW64 (TDM and its bundled libraries).