Difference between C++14 and C++17 using: `*p++ = *p`

Question

While writing some code, I came across a problem where values I set were being set wrong. I eventually found the culprit line and while testing around found that it behaved differently on C++14 and C++17. The code is as follows:

#include <stdio.h> #include <cstdint> #include <cstring>  int main() {     uint8_t *p = new uint8_t[3];     memset(p, 0x00, 1);     p++;     memset(p, 0xF0, 1);     p++;     memset(p, 0xFF, 1);     p--;     p--;      // This line in particular     *p++ = *p;      *p++ = 0x0F;      p--;     p--;      printf("Position 0 has value %u\n", *p);     p++;     printf("Position 1 has value %u\n", *p);     p++;     printf("Position 2 has value %u\n", *p);      return 0; } 

On C++14 it prints:

Position 0 has value 240 Position 1 has value 15 Position 2 has value 255 

And on C++17 it prints:

Position 0 has value 0 Position 1 has value 15 Position 2 has value 255 

I'm curious why it acts differently on different C++ versions. It looks as though on C++14 the *p right side of the assignment is evaluated after the ++. Did this change? And if the ++ has precedence, why does it not happen before the dereference on the left side of the assignment operator?

Answer 1

Reading from and writing to the variable (via the post-increment) used to have undefined behaviour, because the = did not introduce a sequence point. You could have received either behaviour (or none, or explosions) in C++14.

Now, there is a sequencing order defined for this case and your C++17 results are reliable.

Although it's still bad, unclear code that should not be written!