Uniqueness of std::unique_ptr - c++11

Question

I have been working on std::unique_ptr s but confused at some point about its semantics. From the documentation,
No two unique_ptr instances can manage the same object
But, even tough it is most probably a silly example, consider such a code.
std::unique_ptr<int> a(new int(10));
std::unique_ptr<int> b = std::unique_ptr<int>(a.get());
std::cout << *b << std::endl;
*a = 5;
std::cout << *b;
a and b is managing the same object here, and the output is 10 5. And of course I am getting an assertion failure error at the end on debug mode due to two unique ptrs trying to destruct same object at the end of scope.
I know it is silly and such usage is not advised, but I came across to this when it was not very obvious ( a class member calling another etc. ) and the assertion failure was the thing I started with.
My question is what the above sentence exactly means: it is posed by the standard and a decent compiler shouldnt allow you to do it (i am on vs2013 btw) or you must do it that way ( never cause two unique_ptrs point to same object) ( unlikely since the purpose of unique_ptrs is to make us less worried i suppose.) Or I should never use anything about raw pointers ( a.get() ) when unique_ptr s are involved.

Answer 1

Your last sentence is correct. As soon as you use raw pointers with a.get() as in the line below, you have thrown away all the promises that std::unique_ptr make to you.
std::unique_ptr<int> b = std::unique_ptr<int>(a.get());
The correct semantic to preserve the uniqueness while converting to a raw pointer would be to use a.release().
std::unique_ptr<int> b = std::unique_ptr<int>(a.release());
Of course, you would normally just use assignment or initializatoin with std::move if you were moving ownership between two std::unique_pointer instances, as given by the documentation. Either of the two lines below should be valid.
std::unique_ptr<int> b(std::move(a));
std::unique_ptr<int> b = std::move(a);
To make the std::move semantics more clear, consider the following test program.
#include <stdio.h>
#include <memory>
#include <stdlib.h>
int main(){
std::unique_ptr<int> a(new int(10));
printf("%p\n", a.get());
std::unique_ptr<int> b(std::move(a));
printf("%p\n", a.get());
printf("%p\n", b.get());
}
On my system, the output is the following. Observe that the first line and the last line match.
0x1827010
(nil)
0x1827010