这段代码在做什么？ （为size_t）-1-

Question

Can someone explain what happens when size_t, or any other type identifier, is wrapped in parentheses. I know it is the old typecast syntax but in this context I don't follow what is happening.

I've seen it for defining the max size of a type as:

size_t max_size = (size_t)-1

Answer 1

This code (unnecessarily) casts -1 to size_t . The most probable intent was getting the largest possible value of size_t on this system.

Although this code doesn't have Undefined Behavior, this code is ugly - in C++ you should use std::numeric_limits<size_t>::max() and in C use SIZE_MAX macro for exactly a purpose of getting the largest size_t value.

Answer 2

(size_t)-1 is in fact the equivalent of size_t(-1)

See also the following question c cast syntax styles

Answer 3

Some library methods intentionally return (size_t)(-1) to indicate an error condition. For example, the iconv method from the GNU libiconv library . I assume there is some good reason why these functions don't return ssize_t (signed) return values, which would allow you to check for -1 directly.