strange problem in a c++ program with pointers - c++

Question

I wrote this simple c++ program and I got some strange results that I don't understand (results are described in the line comments)
int arr[3] {1, 2, 3};
int* p{ nullptr };
p = arr;
std::cout << p[0] << " " << p[1] << " " << p[2]; // prints 1 2 3, OK
p = arr;
std::cout << *(p++) << " " << *(p++) << " " << *(p); // prints 2 1 3 ??
p = arr;
std::cout << *p << " " << *(++p) << " " << *(++p); // prints 3 3 3 ??
p = arr;
std::cout << *p << " "; ++p;
std::cout << *p << " "; ++p;
std::cout << *p; // prints 1 2 3, OK
it seems that the pointer increments along a std::cout concatenation don't work.
What's wrong in my idea?
I supposed it should have worked.
best
final edit: I was using c++14, I switched to c++20 and now it works properly
thank you everybody!

Answer 1

int* p{ nullptr };
std::cout << p[0] << " " << p[1] << " " << p[2];
This is Undefined Behavior, as you are dereferencing nullptr, p does not point at valid memory yet.
p = arr;
std::cout << p[0] << " " << p[1] << " " << p[2];
This is well-defined behavior. p points at valid memory, is always incremented before dereferenced, and is incremented in a deterministic and valid manner. This is the same as if you had written the following instead:
std::cout << *(p+0) << " " << *(p+1) << " " << *(p+2);
p = arr;
std::cout << *(p++) << " " << *(p++) << " " << *(p);
p = arr;
std::cout << *p << " " << *(++p) << " " << *(++p);
Both of these are Undefined Behavior prior to C++17, because the order in which chained operator<< calls are evaluated is not guaranteed in earlier versions, the compiler is free to evaluate them in whatever order it wants. This is no longer the case in C++17 onward.
p = arr;
std::cout << *p << " "; ++p;
std::cout << *p << " "; ++p;
std::cout << *p;
This is well-defined behavior. p points at valid memory, is always dereferenced before incremented, and is incremented in a deterministic and valid manner.