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For portable applications, use only the assignment = operator, the equality operators == and !=, and the unary & operator on plain character or plain wide character-typed expressions.

Wiki MarkupThis is recommended because the C99 standard requires only the digit characters ('0' - '9') to have consecutive numerical values \[C99 N1401\]. Thus, operations that rely on expected values for plain character or plain wide character-typed expressions can lead to unexpected behavior.

However, due to the requirement for digit characters, the usage of other operators is allowed for them according to the following restrictions:

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This rule appears in the C++ Secure Coding Standard as \[[cplusplus:STR07-CPP. Don't assume numeric values for expressions with type plain character]\].

References

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\[C99 N1401\] Section 5.2.1 "Character sets"