V2568. MISRA. Both operands of an operator should be of the same type category.

This diagnostic rule is based on the software development guidelines developed by MISRA (Motor Industry Software Reliability Association).

This diagnostic rule applies only to code written in C.

The MISRA standard defines an essential type model, where variables can have the following types:

  • Boolean for Boolean values true/false: '_Bool';
  • signed for signed integers or unnamed enums: 'signed char', 'signed short', 'signed int', 'signed long', 'signed long long', 'enum { .... };';
  • unsigned for unsigned integers: 'unsigned char', 'unsigned short', 'unsigned int', 'unsigned long', 'unsigned long long';
  • floating for floating-point values: 'float', 'double', 'long double';
  • character for characters only: 'char';
  • named enum for a named set of user-defined values: 'enum name { .... };'.

This model does not include pointers.

The C language allows much freedom in casting between arithmetic types, but it can also lead to hidden problems such as loss of sign, value, or precision. Despite its strictness, the MISRA standard does allow conversions between arithmetic types when the operands have the same essential types.

Exception: the essential types of the left and right operands of the operators '+', '-', '+=', and '-=' can be 'character' and 'signed' / 'unsigned' respectively.

Example of non-compliant code:

enum { A };
int i;
unsigned u;
void foo()
  A + u;
  0.f - i;
  A > (_Bool)0;

Example of code considered compliant from the viewpoint of this diagnostic:

void foo(unsigned short x, _Bool b)
    x + 1UL;
    if (b && x > 4U) ....

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