C++ most efficient way to convert string to int (faster than atoi)

Question

As mentioned in the title, I\'m looking for something that can give me more performance than atoi. Presently, the fastest way I know is

atoi(mystring.c_str(         


        

Answer 1

Paddy's implementation of fast_atoi is faster than atoi - without the shadow of the doubt - however it works only for unsigned integers.

Below, I put evaluated version of Paddy's fast_atoi that also allows only unsigned integers but speeds conversion up even more by replacing costly operation * with +

unsigned int fast_atou(const char *str)
{
    unsigned int val = 0;
    while(*str) {
        val = (val << 1) + (val << 3) + *(str++) - 48;
    }
    return val;
}

Here, I put complete version of fast_atoi() that i'm using sometimes which converts singed integers as well:

int fast_atoi(const char *buff)
{
    int c = 0, sign = 0, x = 0;
    const char *p = buff;

    for(c = *(p++); (c < 48 || c > 57); c = *(p++)) {if (c == 45) {sign = 1; c = *(p++); break;}}; // eat whitespaces and check sign
    for(; c > 47 && c < 58; c = *(p++)) x = (x << 1) + (x << 3) + c - 48;

    return sign ? -x : x;
}