C and C++ say different things about the result of prefix ++. In C++:
[expr.pre.incr]
The operand of prefix ++ is modified by adding 1. The operand shall be
a modifiable lvalue. The type of the operand shall be an arithmetic
type other than cv bool, or a pointer to a completely-defined object
type. The result is the updated operand; it is an lvalue, and it is a
bit-field if the operand is a bit-field. The expression ++x is
equivalent to x+=1.
So ++ can be applied on the result again, because the result is basically just the object being incremented and is an lvalue. In C however:
6.5.3 Unary operators
The operand of the prefix increment or decrement operator shall have atomic, qualified, or unqualified real or pointer type, and shall be a modifiable lvalue.
The value of the operand of the prefix ++ operator is incremented. The
result is the new value of the operand after incrementation.
The result is not an lvalue; it’s just the pure value of the incrementation. So you can’t apply any operator that requires an lvalue on it, including ++.
If you are ever told the C++ and C are superset or subset of each other, know that it is not the case. There are many differences that make that assertion false.