The C23 edition of Modern C

It should be ++C because with C++ the value you get from the expression is the old one.

If you're asking why people use pre-increment by default instead of post-increment, it's mostly historical. The early C compilers on resource-constrained platforms such as early DOS were not good at optimization; on those, pre-increment would be reliably translated to a simple ADD or INC, whereas code for post-increment might generate an extra copy even if it wasn't actually used.

For C++ this was even worse with iterators, because now it depended on the compiler's ability to inline its implementation of postfix ++, and then prove that all the copies produced by that implementation have no side effects to optimize it to the same degree as prefix ++ could. Depending on the type of the underlying value, this may not even be possible in general.

The other reason is that all other unary operators in C are prefix rather than postfix, and mixing unary prefix with unary postfix in a single expression produces code that is easy to misunderstand. E.g. *p++ is *(p++), not (*p)++, even though the latter feels more natural, reading it left-to-right as usual. OTOH *++p vs ++*p is unambiguous.

K&R seems to use pre-increment early on, then post-increment consistently (or a lot, anyway, I haven't done a thorough check) after chapter 3, in situations where either would do. In fact, after introducing post-increment at 2.8.

> It should be ++C because with C++ the value you get from the expression is the old one.

You get it!