How is a bool represented in memory?

Question

As already discussed in the docs, a bool data type occupies at least a byte of memory. A similar question was asked on SO before (How a bool type variable is stored in memory? (C++)), but this discussion and the documentation only seem to discuss the amount of space occupied by a boolean data type, not what actually happens in memory when I do this:

bool b = true;

So what does actually happen in memory? What happens to the 7 bits that are not used in storing this information? Does the standard prescribe behavior for this?

Are they undefined? Or did someone at C++ headquarters just do this:

enum bool : char 
{
  false = 0,
  true = 1
};

Answer 1

The standard doesn't mandate anything for the binary representation of bools; it just says that, when converting to other integral types, a true bool will become 1 and a false bool will become 0.

This of course suggests an implementation similar in spirit to the one you said, where such conversions would become essentially no-ops or plain integer widening (but remember that bool is mandated to be a primitive type, not an enumeration type).

Answer 2

Standard states that bool values behave as integral types, yet it doesn't specify their concrete representation in memory:

"Values of type bool are either true or false. As described below, bool values behave as integral types. Values of type bool participate in integral promotions" _{~ C++03 3.9.1 §6}

"A synonym for integral type is integer type. The representations of integral types shall define values by use of a pure binary numeration system" _{~ C++03 3.9.1 §7}

Yet it defines the integral promotion from bool to int:

"An rvalue of type bool can be converted to an rvalue of type int, with false becoming zero and true becoming one. These conversions are called integral promotions." _{~ C++03 4.5 §4-5}

as well as conversion from other types to bool:

"A zero value, null pointer value, or null member pointer value is converted to false; any other value is converted to true." _{~ C++03 4.12 §1}