プログラミング言語C/C++における、1文字の数字から1桁の整数値へ変換するイディオム ch - '0' について。
C/C++言語仕様では、数字の文字コード'0'〜'9'がその並び順で連続かつ隣接すると保証する。このため変換元文字chから数字0の文字コード'0'を減算する下記処理は、処理系やソースコードの文字集合によらず正常動作する。*1
int ch = /*...*/; assert(isdigit(ch)); // chは任意の数字 int n = ch - '0'; // 数字から数値へ変換
C99 5.2.1/p3より一部引用（下線部は強調）。
3 Both the basic source and basic execution character sets shall have the following members:
(snip)
the 10 decimal digits0 1 2 3 4 5 6 7 8 9(snip) The representation of each member of the source and execution basic character sets shall fit in a byte. In both the source and execution basic character sets, the value of each character after
0in the above list of decimal digits shall be one greater than the value of the previous. (snip)
C++03 2.2/p1,3より一部引用（下線部は強調）。
1 The basic source character set consists of 96 characters: the space character, the control characters representing horizontal tab, vertical tab, form feed, and new-line, plus the following 91 graphical characters:
a b c d e f g h i j k l m n o p q r s t u v w x y z A B C D E F G H I J K L M N O P Q R S T U V W X Y Z 0 1 2 3 4 5 6 7 8 9 _ { } [ ] # ( ) < > % : ; . ? * + - / ^ & | ~ ! = , \ " '3 (snip) For each basic execution character set, the values of the members shall be non-negative and distinct from one another. In both the source and execution basic character sets, the value of each character after
0in the above list of decimal digits shall be one greater than the value of the previous. (snip)
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