Do negative numbers return false in C/C++?

All non-zero values will be converted to true, and zero values to false. With negative numbers being non-zero, they are converted to true.

Quoting from the C++11 standard (emphasis mine):

4.12 Boolean conversions [conv.bool]

1 A prvalue of arithmetic, unscoped enumeration, pointer, or pointer
to member type can be converted to a prvalue of type bool. A zero
value, null pointer value, or null member pointer value is converted
to false; any other value is converted to true. A prvalue of type
std::nullptr_t can be converted to a prvalue of type bool; the
resulting value is false.

Are they always true/false regardless of compilers?

You will only get the above guarantee when your compiler is standards-compliant, or at least, complies with this specific part of the standard. In practice, all compilers have this standard behavior, so there isn’t much to worry about.