c++ - What is shared_ptr's aliasing constructor for? -

in page (http://www.cplusplus.com/reference/memory/shared_ptr/), paragraph 5, says:

additionally, shared_ptr objects can share ownership on pointer while @ same time pointing object. ability known aliasing (see constructors), , commonly used point member objects while owning object belong to. because of this, shared_ptr may relate 2 pointers:

• a stored pointer, pointer said point to, , 1 dereferences operator*.

• an owned pointer (possibly shared), pointer ownership group in charge of deleting @ point, , counts use.

generally, stored pointer , owned pointer refer same object, but alias shared_ptr objects (those constructed alias constructor , copies) may refer different objects.

then read page (http://www.cplusplus.com/reference/memory/shared_ptr/shared_ptr/) aliasing constructor of shared_ptr. still think "aliasing" behavior confusing. why here? for? in situation want feature?

simple example:

struct bar {      // data want point };  struct foo {     bar bar; };  shared_ptr<foo> f = make_shared<foo>(some, args, here); shared_ptr<bar> specific_data(f, &f->bar);  // ref count of object pointed f 2 f.reset();  // foo still exists (ref cnt == 1) // our bar pointer still valid, , can use stuff some_func_that_takes_bar(specific_data); 

aliasing when want point bar, don't want foo deleted out under us.

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as johannes points out in comments, there equivalent language feature:

const bar& specific_data = foo(...).bar; 

we're taking reference member of temporary, temporary foo still kept alive long specific_data is. shared_ptr example, have bar lifetime tied foo - foo cannot access.