Question

I have a question concerning the difference between a “normal” C++ string and a string of unsigned characters.
When generating some pseudorandom strings of chars and of unsigned chars, I noticed a huge performance difference between the time the code would need to build a normal string and one composed of unsigned chars.

The code I used:

#include <tr1/random>
#include <string>


using namespace std;
using namespace tr1;

typedef basic_string<unsigned char > ustring;

string generateString(){
    string retStr;
    char a;

    for(unsigned int i = 0; i < 1000; i++){             
        a = rand();
        retStr+=a;            
    }
    return retStr;
}

ustring generateUString(){
    ustring retStr;
    unsigned char a;

    for(unsigned int i = 0; i < 1000; i++){
        a = rand();
        retStr+=a;
    }
    return retStr;
}


int main(int argc, char* args[]){

    srand(0);

    string thing;
    ustring uthing;

    for(unsigned int i = 1; i< 100000; i++){ 
        //thing = generateString(); // this needs 2 second to execute
        uthing = generateUString(); //  and this 13

    }

    return 0;
}

So basically, the code needs 2 seconds to execute generateString() 100 000 times, while it needs 13 seconds to execute generateUString() 100 000 times.

What exactly is the reason for this? I guess it’s the += operator, since the difference melted away when I cut the corresponding lines (actually, generateUstring() seems to be faster then, I guess because the modulo arithmetic is easier in that case).

But why then is it so much faster to append a char to a string than to append a unsigned char to a string of unsigned chars? And should I hence avoid strings of unsigned chars?

Answer 1

The reason is probably that basic_string<char> is explicitly instantiated in libstdc++.so which (by default) is compiled with -O2. So if you don’t compile your program with optimization the basic_string<unsigned char> operations will be un-optimized, but all the basic_string<char> operations that aren’t inlined will use the optimized code in libstdc++.so.