bool to int conversion

int x = 4<5;

Completely portable. Standard conformant. bool to int conversion is implicit!

§4.7/4 from the C++ 11 or 14 Standard, §7.8/4 from the C++ 17 Standard, §7.3.9/2 from the 20 Standard says (Integral Conversion)

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

As for C, as far as I know there is no bool in C. (before 1999) So bool to int conversion is relevant in C++ only. In C, 4<5 evaluates to int value, in this case the value is 1, 4>5 would evaluate to 0.

EDIT: Jens in the comment said, C99 has _Bool type. bool is a macro defined in stdbool.h header file. true and false are also macro defined in stdbool.h.

§7.16 from C99 says,

The macro bool expands to _Bool.

[..] true which expands to the integer constant 1, false
which expands to the integer constant 0,[..]