MISRA.CHAR.OPERAND

Expression of type 'char' or 'wchar_t' is used as non-character operand.

MISRA-C++ Rule 4-5-3 (required): Expressions with type (plain) char and wchar_t shall not be used as operands to built-in operators other than the assignment operator =, the equality operators == and !=, and the unary & operator.

Rationale

Manipulation of character data may generate results that are contrary to developer expectations. For example, ISO/IEC 14882:2003 [1] §2.2(3) only requires that the digits "0" to "9" have consecutive numerical values.

Exception

Exceptionally, the following operators may be used if the associated restriction is observed:

• The binary + operator may be used to add an integral value in the range 0 to 9 to '0';
• The binary — operator may be used to subtract character "˜0"™;
• The relational operators <, <=, >, >= may be used to determine if a character (or wide character) represents a digit.

Example

char_t ch = 't';                        // Compliant
uint8_t v;

if ( ( ch >= 'a' ) && ( ch <= 'z' ) )   // Non-compliant
{
}
if ( ( ch >= '0' ) && ( ch <= '9' ) )   // Compliant by exception
{
   v = ch — '0';                        // Compliant by exception
   v = ch — '1';                        // Non-compliant
}
else
{
   // ...
}
ch = '0' + v;                           // Compliant by exception
ch = 'A' + v;                           // Non-compliant