You can utilize the fact that the character encodings for digits are all in order from 48 (for ‘0’) to 57 (for ‘9’). This holds true for ASCII, UTF-x and practically all other encodings (see comments below for more on this).
Therefore the integer value for any digit is the digit minus ‘0’ (or 48).
char c="1";
int i = c - '0'; // i is now equal to 1, not '1'
is synonymous to
char c="1";
int i = c - 48; // i is now equal to 1, not '1'
However I find the first c - '0' far more readable.