26. Insidious VARIANT_BOOL

The fragment is taken from NAME project. The code contains an error that PVS-Studio analyzer diagnoses in the following way: V721 The VARIANT_BOOL type is utilized incorrectly. The true value (VARIANT_TRUE) is defined as -1. Inspect the first argument.

virtual HRESULT __stdcall
  put_HandleKeyboard (VARIANT_BOOL pVal) = 0;
....
pController->put_HandleKeyboard(true);

Explanation:

There is quite a witty quote:

We all truck around a kind of original sin from having learned Basic at an impressionable age. (C) P.J. Plauger

And this hint is exactly on the topic of evil. VARIANT_BOOL type came to us from Visual Basic. Some of our present day programming troubles are connected with this type. The thing is that "true" is coded as -1 in it.

Let's see the declaration of the type and the constants denoting true/false:

typedef short VARIANT_BOOL;

#define VARIANT_TRUE ((VARIANT_BOOL)-1)

#define VARIANT_FALSE ((VARIANT_BOOL)0)

It seems like there is nothing terrible in it. False is 0, and truth is not 0. So, -1 is quite a suitable constant. But it's very easy to make an error by using true or TRUE instead of VARIANT_TRUE.

Correct code

pController->put_HandleKeyboard(VARIANT_TRUE);

Recommendation

If you see an unknown type, it's better not to hurry, and to look up in the documentation. Even if the type name has a word BOOL, it doesn't mean that you can place 1 into the variable of this type.

In the same way programmers sometimes make mistakes, when they use HRESULT type, trying to compare it with FALSE or TRUE and forgetting that:

#define S_OK     ((HRESULT)0L)
#define S_FALSE  ((HRESULT)1L)

So I really ask you to be very careful with any types which are new to you, and not to hasten when programming.