Mozilla C/C++ coding style

Question

Today I was going through the "Mozilla Coding Style Guideline" and I found a point I am not sure about.

What do they mean with the following statement?

Do not compare x == true or x == false. Use (x) or (!x) instead. x == true, in fact, is different from if (x)!

I am not sure how these two methods are different. Please clarify.

Answer 1

Evaluation of x == true and/or x == false depends on the definitions of true and false, which may not be universally the same.

(x) and (!x) are evaluated based on the value of x only and does not depend on any platform / implementation / standard specific definitions. For example, (x) will be evaluated to produce a TRUE result in case of x having a non-zero value.

In C, as per C11 standard, stdbool.h header, chapter 7.18 the definition of true and false are

true

which expands to the integer constant 1,

false

which expands to the integer constant 0

so, in case of x having a value 2

• x == true will be evaluated to produce FALSE.
• (x) will produce TRUE.

So, to produce a robust code and maintain portability, it's better to use the later approach.