Is !! a safe way to convert to bool in C++?

Question

[This question is related to but not the same as this one.]

If I try to use values of certain types as boolean expressions, I get a warning. Rather than su

Answer 1

!! is only useful when you're using a boolean expression in arithmetic fashion, e.g.:

c = 3 + !!extra; //3 or 4

(Whose style is a different discussion.) When all you need is a boolean expression, the !! is redundant. Writing

bool b = !!extra;

makes as much sense as:

if (!!extra) { ... }