[C++11] shared_ptr alias constructor

std::shared_ptr's secret constructor

struct X{
    Y y;
};


struct do_nothing_deleter{
    template<typename> void operator()(T*){}
};


void store_for_later(std::shared_ptr<Y>);


void foo(){
    std::shared_ptr<X> px(std::make_shared<X>());
    std::shared_ptr<Y> py(&px->y,do_nothing_deleter());
    store_for_later(py);
} // our X object is destroyed, BAD!

void bar(){
    std::shared_ptr<X> px(std::make_shared<X>());
    std::shared_ptr<Y> py(px,&px->y);
    store_for_later(py);
} // our X object is kept alive, i.e px NEVER destroyed even out of scope.


struct X2{
    std::unique_ptr<Y> y;
    X2():y(new Y){}
};


void baz(){
    std::shared_ptr<X2> px(std::make_shared<X2>());
    std::shared_ptr<Y> py(px,px->y.get());
    store_for_later(py);
} // our X2 object is kept alive, DITTO

aa as

http://stackoverflow.com/a/27109774/1316609
However, r-reference to prvalue's data member will exist
even the prvalue destructs the internal data member(i.e r-reference make a copy out of it),
same as const T&.

For const T& to a l-value data member, the destruct of internal data member will
effect the outer const T&, this is by standard.
http://stackoverflow.com/a/3097861