Your example doesn't tell me which I have. It's assuming that err = 0 means success, but that's an assumption not a contract of the type.

By comparison some something like https://github.com/oktal/result will tell you if it's a success or error without any magic error codes that actually mean success.

You could do this with a type bit + union in C, it's just more painful without templates

That requires 1 bit of information, and there is often a way of embedding that bit in the value itself; but the general technique of representing alternatives is called “a tagged union.” (You can also hear the fancy terms “the product type” for the struct and “the sum type” for the union.)

Accessing that one bit can be as expensive as passing an int around.