Question

When should BOOL and bool be used in C++ and why?

I think using bool is cleaner and more portable because it’s a built-in type. But BOOL is unavoidable when you interactive with legacy code/C code, or doing inter-op from .NET with C code/Windows API.

So my policy is:
Use bool inside C++.
Use BOOL when talk to outer world, e.g., export function in windows DLL.

Is there a definitive explanation of when to use one over the other?

Answer 1

Matthew Wilson discusses BOOL, bool, and similar in section 13.4.2 of Imperfect C++. Mixing the two can be problematic, since they generally have different sizes (and so pointers and references aren’t interchangeable), and since bool isn’t guaranteed to have any particular size. Trying to use typedefs or conditional compilating to smooth over the differences between BOOL and bool or trying to allow for a single Boolean type to work in both C and C++ is even worse:

#if defined(__cplusplus) || \
    defined(bool) /* for C compilation with C99 bool (macro) */
 typedef bool   bool_t;
#else
 typedef BOOL   bool_t;
#endif /* __cplusplus */

This approach means that a function’s return type can differ depending on which language calls it; Wilson explains that he’s seen more than one bug in his own code and others’ that results from this. He concludes:

The solution to this imperfection is, as it so often is, abstinence. I never use bool for anything that can possibly be accessed across multiple link units—dynamic/static libraries, supplied object files—which basically means not in functions or classes that appear outside of header files. The practical answer, such as it is, is to use a pseudo-Boolean type, which is the size of int.

In short, he would agree with your approach.