在带有指针变量的表达式右侧使用逻辑非运算符（两次）

Question

Why would anyone type variable = !!ptr as an expression? Looks like a bug or unintentional defect to me. The result should be just ptr , but one must wonder the original intent. Thoughts?

Answer 1

The ! operator results in a value of 0 if its operand is equal to 0, or 1 otherwise.

If ptr is 0 (or NULL if it's a pointer), then !ptr will evaluate to 1, and !!ptr will evaluate to 0. If ptr is not 0 (or not NULL), then !ptr will evaluate to 0 and !!ptr will evaluate to 1.

So the end result of !!ptr is that the value of ptr is normalized to a boolean value, ie 0 will remain 0 and non-zero will be converted to 1.

Answer 2

For all scalar types, !x is equivalent to x == 0 and !!x is equivalent to x != 0 . !!x is hence an idiom to normalize x as a boolean.

If the type of variable is bool , variable = !!ptr is indeed equivalent to variable = ptr and will generate the same code.

Which one is more readable is debatable: implicit conversion to bool is somewhat confusing and error prone because variable = ptr would mean something totally different if variable has a different integer type, whereas !!ptr is safe and explicit, once you master the idiom.