Why are multiple increments/decrements valid in C++ but not in C?

Question

test.(c/cpp)

#include <stdio.h>

int main(int argc, char** argv)
{
  int a = 0, b = 0;
  printf("a = %d, b = %d\n", a, b);
  b = (++a)--;
  printf("a = %d, b = %d\n", a, b);

  return 0;
}

If I save the above as a .cpp file, it compiles and outputs this upon execution:

a = 0, b = 0
a = 0, b = 1

However, if I save it as a .c file, I get the following error:

test.c:7:12: error: lvalue required as decrement operator.

Shouldn't the (++a) operation be resolved before the (newValue)-- operation? Does anyone have any insight on this?

Answer 1

In C the result of the prefix and postfix increment/decrement operators is not an lvalue.

In C++ the result of the postfix increment/decrement operator is also not an lvalue but the result of the prefix increment/decrement operator is an lvalue.

Now doing something like (++a)-- in C++ is undefined behavior because you are modifying an object value twice between two sequence points.

EDIT: following up on @bames53 comment. It is undefined behavior in C++98/C++03 but the changes in C++11 on the idea of sequence points now makes this expression defined.