strange problem in a c program with pointers-CodePudding

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!

CodePudding user response：

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.