Before bool became part of Standard C++, everyone tended to use different techniques in order to produce Boolean-like behavior. These produced portability problems and could introduce subtle errors.

The Standard C++ bool type can have two states expressed by the built-in constants true (which converts to an integral one) and false (which converts to an integral zero). All three names are keywords. In addition, some language elements have been adapted:

Because there’s a lot of existing code that uses an int to represent a flag, the compiler will implicitly convert from an int to a bool (nonzero values will produce true while zero values produce false). Ideally, the compiler will give you a warning as a suggestion to correct the situation.

An idiom that falls under “poor programming style” is the use of ++ to set a flag to true. This is still allowed, but deprecated, which means that at some time in the future it will be made illegal. The problem is that you’re making an implicit type conversion from bool to int, incrementing the value (perhaps beyond the range of the normal bool values of zero and one), and then implicitly converting it back again.

Pointers (which will be introduced later in this chapter) will also be automatically converted to bool when necessary.