Questions about pointers

[Robertlzw]

The following code gives an output of 4111

int list[] = { 1, 2, 3, 4, 5, 6, 7};
int *p = list;
std::cout << *(p+3) << *++p << *--p << *p << std::endl;

I have difficulty in understanding the 2nd and 3rd 1. Can someone explain it? Thanks in advance.

[Disch]

What you're doing is sort of illegal and has undefined output, which is why it's hard to understand.

You can't modify a variable (p) and read it multiple times in the same instruction because the compiler is free to rearrange the order of those writes/reads however it sees fit.

Try this instead:

std::cout << *(p+3);
std::cout << *++p;
std::cout << *--p;
std::cout << *p << std::endl;

[Galik]

What you are doing has undefined behaviour. The compiler is not allowed to modify a variable more than once between sequence points:

http://www.parashift.com/c++-faq-lite/misc-technical-issues.html#faq-39.15

[Robertlzw]

Actually it was an interview question I was asked many years ago. Another such an illegal example is:

++++p; 

where p is a pointer.

[naderST]

Why ++++p is illegal?