In C++11, operator<  for unique_ptr [unique.ptr.special] is defined as

std::less<CT>()(x.get(), y.get())

where CT is the common_type of the unique_ptr's internal pointer typedefs.


On the other hand, operator<  for shared_ptr [util.smartptr.shared.cmp]
is defined as

std::less<V>()(a.get(), b.get())

"where V is the composite pointer type (5.9) of T* and U*."


What exactly is the difference, and why?


Bo Persson


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On 2012-05-28 23:53, Bo Persson wrote:
>  In C++11, operator<  for unique_ptr [unique.ptr.special] is defined as
>
>  std::less<CT>()(x.get(), y.get())
>
>  where CT is the common_type of the unique_ptr's internal pointer typedefs.
>
>
>  On the other hand, operator<  for shared_ptr [util.smartptr.shared.cmp]
>  is defined as
>
>  std::less<V>()(a.get(), b.get())
>
>  "where V is the composite pointer type (5.9) of T* and U*."
>
>
>  What exactly is the difference, and why?

The wording difference exists, because std::unique_ptr can hold both
built-in pointers and pointer-like types while std::shared_ptr can only
hold pointers. The intention is that unique_ptr's wording has exactly
the same effect when it holds a pointer. Currently these definitions are
not the same because of a wording defect of std::common_type. An issue
has already been raised for this but is not yet visible in the official
list. In short words, that defect is because std::common_type<int*,
int*>::type evaluates to int*&&  and not int*. This was an unintended
side-effect because of change of the original definition of the
conditional operator in the presence of xvalues.

HTH&  Greetings from Bremen,

Daniel Kr   gler


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