What's the fastest way to convert hex to integer in C++?

Question

I\'m trying to convert a hex char to integer as fast as possible.

This is only one line: int x = atoi(hex.c_str);

Is there a faster way

Answer 1

Assuming that your function is called for a valid hex digit, it will cost in average at least 8 comparison operations (and perhap's 7 jumps). Pretty expensive.

An alternative would be the more compact:

if (number >= '0' && number<='9') 
    return number-'0';
else if (number >= 'a' && number <='f') 
    return number-'a'+0x0a; 
else return -1; 

Yet another alternative would be to use a lookup table (trade space against speed), that you would initialise only once, and then access directly:

if (number>=0) 
   return mytable[number]; 
else return -1;   

If you want to convert more than one digit at a time, you could have a look at this question)

Edit: benchmark

Following Ike's observations, Ive written a small informal benchmark (available online here), that you can run on your favourite compiler.

Conclusions:

• The lookup table is always the winner
• The switch better than the if-chain.
• With msvc2015 (release), the second best is my compact version, surprisingly followed closely by the map version of 101010.
• With gcc on ideone, the second is the switch version followed by the compact version.