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);
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.
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.