What happens to char round-trip cast through a bool?

Question

What does the C++ language definition promise about casting a char to bool then back to char again?

char original = 255;
bool next = original;
char final = next;

Also, what do most compilers do in this case beyond what the language guarantees?

Answer 1

This will give a value of zero or one, depending on whether the original value was zero or non-zero.

Converting to bool gives a value of true or false:

4.12 A zero value, null pointer value, or null member pointer value is converted to false; any other value is converted to true.

Converting back to char converts false to zero, and true to one:

4.7/4 If the source type is bool, the value false is converted to zero and the value true is converted to one.

Answer 2

Integral values converted to bool result in either true or false (4.12), and bool converted to integral values results in either 1 or 0 (4.5(6)). See Chapter 4 (Standard Conversions).