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

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I experimented with solutions using lookup tables, but found them fraught with issues, and actually not very fast. The fastest solution turned out to be the least imaginitive:

int fast_atoi( const char * str )
{
    int val = 0;
    while( *str ) {
        val = val*10 + (*str++ - '0');
    }
    return val;
}

Running a benchmark with a million randomly generated strings:

fast_atoi : 0.0097 seconds
atoi      : 0.0414 seconds

To be fair, I also tested this function by forcing the compiler not to inline it. The results were still good:

fast_atoi : 0.0104 seconds
atoi      : 0.0426 seconds

Provided your data conforms to the requirements of the fast_atoi function, that is pretty reasonable performance. The requirements are:

  1. Input string contains only numeric characters, or is empty
  2. Input string represents a number from 0 up to INT_MAX

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