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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.

This 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, because of the requirement for digit characters, other operators can be used for them according to the following restrictions:

  • The binary + operator may be used to add integer values from 0 to 9 to '0'.
  • The binary - operator may be used to subtract character '0'.
  • Relational operators <, <=, >, and >= can be used to check whether a character or wide character is a digit.

Character types should be chosen and used in accordance with recommendation STR04-C. Use plain char for characters in the basic character set.

Noncompliant Code Example

This noncompliant code example attempts to determine if the value of a character variable is between 'a' and 'c' inclusive. However, since the C99 standard does not require the letter characters to be consecutive or in alphabetical order, the check might not work as expected.

char ch = 'b';
if ((ch >= 'a') && (ch <= 'c')) {
  /* ... */

Compliant Solution

In this example, the specific check is enforced using compliant operations on character expressions.

char ch = 't';
if ((ch == 'a') || (ch == 'b') || (ch == 'c')) {
  /* ... */


STR09-EX1: Consecutive values for characters like a~z can be assumed on platforms where ASCII or Unicode is used. This recommendation is primarily concerned with platform portability, for example, if code is migrated from ASCII systems to non-ASCII systems.

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Related Guidelines

CERT C++ Secure Coding Standard: STR07-CPP. Don't assume numeric values for expressions with type plain character


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

      07. Characters and Strings (STR)      

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