This constructor is so secret, not even STL maintainers know about it ...

Stephan T. Lavavej

This constructor is so secret that even accompanying STL do not know about it ...

lane: Door

std::shared_ptr has a small secret: a very useful constructor that most programmers haven’t even heard about. It was added only in standard C ++ 11, and it was not even in the TR1 version of shared_ptr . However, it is supported by gcc from version 4.3, and by the MSVC compiler from the time of Visual Studio 2010. It appeared in Boost from about 1.35.0.

Most of the tutorials that describe std::shared_ptr have nothing about this constructor. Scott Myers did not say a word about him in Effective Modern C ++, another author Nicolai Josuttis devoted to this designer about half a page in his book The C ++ Standard Library.
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So what is this secret constructor?

This is a pseudonymous constructor (aliasing constructor).

Aliases shared_ptr

What does this constructor do? It allows you to create shared_ptr , which shares ownership with another shared_ptr , but (attention!) Has a different pointer. And I do not mean a pointer of another type, which was simply cast from one type to another, I am talking about a completely different value of the pointer. That is, you can have both shared_ptr<std::string> and shared_ptr, ( - ).

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, X, Y,

struct X{ Y y; };
, shared_ptr, px px->y , shared_ptr . : shared_ptr, , , , Y. , , shared_ptr , px ?

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); } // X
shared_ptr , , , . , . , , , shared_ptr shared_ptr. shared_ptr .

void bar(){ std::shared_ptr<X> px(std::make_shared<X>()); std::shared_ptr<Y> py(px,&px->y); store_for_later(py); } // X
. - , shared_ptr . X2, Y,

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); } // X2
, pimpl . , , , . . X , shared_ptr, shared_ptr , shared_ptr (.. reset() ).


:

template<typename Other,typename Target> shared_ptr(shared_ptr<Other> const& other,Target* p);
shared_ptr p * , . Other . other , other.use_count() 1. , get() - static_cast<T*>(p) .

: other shared_ptr , , , shared_ptr , use_count() == 0 . , p NULL.

int i; shared_ptr<int> sp(shared_ptr<X>(),&i); assert(sp.use_count()==0); assert(sp.get()==&i);
.


shared_ptr , .

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reddit
cppreference at the same time shared_ptr, ( - ).

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, X, Y,

struct X{ Y y; };
, shared_ptr, px px->y , shared_ptr . : shared_ptr, , , , Y. , , shared_ptr , px ?

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); } // X
shared_ptr , , , . , . , , , shared_ptr shared_ptr. shared_ptr .

void bar(){ std::shared_ptr<X> px(std::make_shared<X>()); std::shared_ptr<Y> py(px,&px->y); store_for_later(py); } // X
. - , shared_ptr . X2, Y,

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); } // X2
, pimpl . , , , . . X , shared_ptr, shared_ptr , shared_ptr (.. reset() ).


:

template<typename Other,typename Target> shared_ptr(shared_ptr<Other> const& other,Target* p);
shared_ptr p * , . Other . other , other.use_count() 1. , get() - static_cast<T*>(p) .

: other shared_ptr , , , shared_ptr , use_count() == 0 . , p NULL.

int i; shared_ptr<int> sp(shared_ptr<X>(),&i); assert(sp.use_count()==0); assert(sp.get()==&i);
.


shared_ptr , .

, , .

reddit
cppreference shared_ptr, ( - ).

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, ? shared_ptr , ( , ) .


, X, Y,

struct X{ Y y; };
, shared_ptr, px px->y , shared_ptr . : shared_ptr, , , , Y. , , shared_ptr , px ?

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); } // X
shared_ptr , , , . , . , , , shared_ptr shared_ptr. shared_ptr .

void bar(){ std::shared_ptr<X> px(std::make_shared<X>()); std::shared_ptr<Y> py(px,&px->y); store_for_later(py); } // X
. - , shared_ptr . X2, Y,

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); } // X2
, pimpl . , , , . . X , shared_ptr, shared_ptr , shared_ptr (.. reset() ).


:

template<typename Other,typename Target> shared_ptr(shared_ptr<Other> const& other,Target* p);
shared_ptr p * , . Other . other , other.use_count() 1. , get() - static_cast<T*>(p) .

: other shared_ptr , , , shared_ptr , use_count() == 0 . , p NULL.

int i; shared_ptr<int> sp(shared_ptr<X>(),&i); assert(sp.use_count()==0); assert(sp.get()==&i);
.


shared_ptr , .

, , .

reddit
cppreference

shared_ptr, ( - ).

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, ? shared_ptr , ( , ) .


, X, Y,

struct X{ Y y; };
, shared_ptr, px px->y , shared_ptr . : shared_ptr, , , , Y. , , shared_ptr , px ?

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); } // X
shared_ptr , , , . , . , , , shared_ptr shared_ptr. shared_ptr .

void bar(){ std::shared_ptr<X> px(std::make_shared<X>()); std::shared_ptr<Y> py(px,&px->y); store_for_later(py); } // X
. - , shared_ptr . X2, Y,

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); } // X2
, pimpl . , , , . . X , shared_ptr, shared_ptr , shared_ptr (.. reset() ).


:

template<typename Other,typename Target> shared_ptr(shared_ptr<Other> const& other,Target* p);
shared_ptr p * , . Other . other , other.use_count() 1. , get() - static_cast<T*>(p) .

: other shared_ptr , , , shared_ptr , use_count() == 0 . , p NULL.

int i; shared_ptr<int> sp(shared_ptr<X>(),&i); assert(sp.use_count()==0); assert(sp.get()==&i);
.


shared_ptr , .

, , .

reddit
cppreference

shared_ptr, ( - ).

, , shared_ptr' . , , , .

, ? shared_ptr , ( , ) .


, X, Y,

struct X{ Y y; };
, shared_ptr, px px->y , shared_ptr . : shared_ptr, , , , Y. , , shared_ptr , px ?

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); } // X
shared_ptr , , , . , . , , , shared_ptr shared_ptr. shared_ptr .

void bar(){ std::shared_ptr<X> px(std::make_shared<X>()); std::shared_ptr<Y> py(px,&px->y); store_for_later(py); } // X
. - , shared_ptr . X2, Y,

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); } // X2
, pimpl . , , , . . X , shared_ptr, shared_ptr , shared_ptr (.. reset() ).


:

template<typename Other,typename Target> shared_ptr(shared_ptr<Other> const& other,Target* p);
shared_ptr p * , . Other . other , other.use_count() 1. , get() - static_cast<T*>(p) .

: other shared_ptr , , , shared_ptr , use_count() == 0 . , p NULL.

int i; shared_ptr<int> sp(shared_ptr<X>(),&i); assert(sp.use_count()==0); assert(sp.get()==&i);
.


shared_ptr , .

, , .

reddit
cppreference

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 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); } //  X  
  shared_ptr     , ,  , .    ,    .  ,    ,    ,     shared_ptr  shared_ptr.  shared_ptr    . 
 
 void bar(){ std::shared_ptr<X> px(std::make_shared<X>()); std::shared_ptr<Y> py(px,&px->y); store_for_later(py); } //  X    
       .   -          ,     shared_ptr .       X2,     Y,         
 
 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); } //  X2    
       ,    pimpl .   ,         ,   ,    .              .    X        ,     shared_ptr,     shared_ptr      ,  shared_ptr      (..   reset() ). 
 
  
  : 
 
 template<typename Other,typename Target> shared_ptr(shared_ptr<Other> const& other,Target* p); 
     shared_ptr  p    * ,    .    Other   .       other ,   other.use_count()   1. ,   get()   -  static_cast<T*>(p) . 
 
   :  other   shared_ptr , ,    ,   shared_ptr   ,    use_count() == 0 .      ,  p    NULL. 
 
 int i; shared_ptr<int> sp(shared_ptr<X>(),&i); assert(sp.use_count()==0); assert(sp.get()==&i); 
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      reddit 
    cppreference 

shared_ptr, ( - ).

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, X, Y,

struct X{ Y y; };
, shared_ptr, px px->y , shared_ptr . : shared_ptr, , , , Y. , , shared_ptr , px ?

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); } // X
shared_ptr , , , . , . , , , shared_ptr shared_ptr. shared_ptr .

void bar(){ std::shared_ptr<X> px(std::make_shared<X>()); std::shared_ptr<Y> py(px,&px->y); store_for_later(py); } // X
. - , shared_ptr . X2, Y,

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); } // X2
, pimpl . , , , . . X , shared_ptr, shared_ptr , shared_ptr (.. reset() ).


:

template<typename Other,typename Target> shared_ptr(shared_ptr<Other> const& other,Target* p);
shared_ptr p * , . Other . other , other.use_count() 1. , get() - static_cast<T*>(p) .

: other shared_ptr , , , shared_ptr , use_count() == 0 . , p NULL.

int i; shared_ptr<int> sp(shared_ptr<X>(),&i); assert(sp.use_count()==0); assert(sp.get()==&i);
.


shared_ptr , .

, , .

reddit
cppreference

 shared_ptr,        (    -     ). 
 
 ,     ,     shared_ptr'   .       ,   ,  ,      . 
 
        ,      ?     shared_ptr ,     ( , )       . 
 
    
 ,    X,    Y,      
 
 struct X{ Y y; }; 
  ,      shared_ptr, px            px->y ,    shared_ptr .     :  shared_ptr,      ,    ,    ,        Y.     ,    ,  shared_ptr   ,  px    ? 
 
 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); } //  X  
  shared_ptr     , ,  , .    ,    .  ,    ,    ,     shared_ptr  shared_ptr.  shared_ptr    . 
 
 void bar(){ std::shared_ptr<X> px(std::make_shared<X>()); std::shared_ptr<Y> py(px,&px->y); store_for_later(py); } //  X    
       .   -          ,     shared_ptr .       X2,     Y,         
 
 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); } //  X2    
       ,    pimpl .   ,         ,   ,    .              .    X        ,     shared_ptr,     shared_ptr      ,  shared_ptr      (..   reset() ). 
 
  
  : 
 
 template<typename Other,typename Target> shared_ptr(shared_ptr<Other> const& other,Target* p); 
     shared_ptr  p    * ,    .    Other   .       other ,   other.use_count()   1. ,   get()   -  static_cast<T*>(p) . 
 
   :  other   shared_ptr , ,    ,   shared_ptr   ,    use_count() == 0 .      ,  p    NULL. 
 
 int i; shared_ptr<int> sp(shared_ptr<X>(),&i); assert(sp.use_count()==0); assert(sp.get()==&i); 
          . 
 
     
            shared_ptr ,                . 
 
 ,     ,          . 
 
      reddit 
    cppreference 

shared_ptr, ( - ).

, , shared_ptr' . , , , .

, ? shared_ptr , ( , ) .


, X, Y,

struct X{ Y y; };
, shared_ptr, px px->y , shared_ptr . : shared_ptr, , , , Y. , , shared_ptr , px ?

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); } // X
shared_ptr , , , . , . , , , shared_ptr shared_ptr. shared_ptr .

void bar(){ std::shared_ptr<X> px(std::make_shared<X>()); std::shared_ptr<Y> py(px,&px->y); store_for_later(py); } // X
. - , shared_ptr . X2, Y,

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); } // X2
, pimpl . , , , . . X , shared_ptr, shared_ptr , shared_ptr (.. reset() ).


:

template<typename Other,typename Target> shared_ptr(shared_ptr<Other> const& other,Target* p);
shared_ptr p * , . Other . other , other.use_count() 1. , get() - static_cast<T*>(p) .

: other shared_ptr , , , shared_ptr , use_count() == 0 . , p NULL.

int i; shared_ptr<int> sp(shared_ptr<X>(),&i); assert(sp.use_count()==0); assert(sp.get()==&i);
.


shared_ptr , .

, , .

reddit
cppreference

 shared_ptr,        (    -     ). 
 
 ,     ,     shared_ptr'   .       ,   ,  ,      . 
 
        ,      ?     shared_ptr ,     ( , )       . 
 
    
 ,    X,    Y,      
 
 struct X{ Y y; }; 
  ,      shared_ptr, px            px->y ,    shared_ptr .     :  shared_ptr,      ,    ,    ,        Y.     ,    ,  shared_ptr   ,  px    ? 
 
 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); } //  X  
  shared_ptr     , ,  , .    ,    .  ,    ,    ,     shared_ptr  shared_ptr.  shared_ptr    . 
 
 void bar(){ std::shared_ptr<X> px(std::make_shared<X>()); std::shared_ptr<Y> py(px,&px->y); store_for_later(py); } //  X    
       .   -          ,     shared_ptr .       X2,     Y,         
 
 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); } //  X2    
       ,    pimpl .   ,         ,   ,    .              .    X        ,     shared_ptr,     shared_ptr      ,  shared_ptr      (..   reset() ). 
 
  
  : 
 
 template<typename Other,typename Target> shared_ptr(shared_ptr<Other> const& other,Target* p); 
     shared_ptr  p    * ,    .    Other   .       other ,   other.use_count()   1. ,   get()   -  static_cast<T*>(p) . 
 
   :  other   shared_ptr , ,    ,   shared_ptr   ,    use_count() == 0 .      ,  p    NULL. 
 
 int i; shared_ptr<int> sp(shared_ptr<X>(),&i); assert(sp.use_count()==0); assert(sp.get()==&i); 
          . 
 
     
            shared_ptr ,                . 
 
 ,     ,          . 
 
      reddit 
    cppreference 

shared_ptr, ( - ).

, , shared_ptr' . , , , .

, ? shared_ptr , ( , ) .


, X, Y,

struct X{ Y y; };
, shared_ptr, px px->y , shared_ptr . : shared_ptr, , , , Y. , , shared_ptr , px ?

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); } // X
shared_ptr , , , . , . , , , shared_ptr shared_ptr. shared_ptr .

void bar(){ std::shared_ptr<X> px(std::make_shared<X>()); std::shared_ptr<Y> py(px,&px->y); store_for_later(py); } // X
. - , shared_ptr . X2, Y,

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); } // X2
, pimpl . , , , . . X , shared_ptr, shared_ptr , shared_ptr (.. reset() ).


:

template<typename Other,typename Target> shared_ptr(shared_ptr<Other> const& other,Target* p);
shared_ptr p * , . Other . other , other.use_count() 1. , get() - static_cast<T*>(p) .

: other shared_ptr , , , shared_ptr , use_count() == 0 . , p NULL.

int i; shared_ptr<int> sp(shared_ptr<X>(),&i); assert(sp.use_count()==0); assert(sp.get()==&i);
.


shared_ptr , .

, , .

reddit
cppreference

 shared_ptr,        (    -     ). 
 
 ,     ,     shared_ptr'   .       ,   ,  ,      . 
 
        ,      ?     shared_ptr ,     ( , )       . 
 
    
 ,    X,    Y,      
 
 struct X{ Y y; }; 
  ,      shared_ptr, px            px->y ,    shared_ptr .     :  shared_ptr,      ,    ,    ,        Y.     ,    ,  shared_ptr   ,  px    ? 
 
 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); } //  X  
  shared_ptr     , ,  , .    ,    .  ,    ,    ,     shared_ptr  shared_ptr.  shared_ptr    . 
 
 void bar(){ std::shared_ptr<X> px(std::make_shared<X>()); std::shared_ptr<Y> py(px,&px->y); store_for_later(py); } //  X    
       .   -          ,     shared_ptr .       X2,     Y,         
 
 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); } //  X2    
       ,    pimpl .   ,         ,   ,    .              .    X        ,     shared_ptr,     shared_ptr      ,  shared_ptr      (..   reset() ). 
 
  
  : 
 
 template<typename Other,typename Target> shared_ptr(shared_ptr<Other> const& other,Target* p); 
     shared_ptr  p    * ,    .    Other   .       other ,   other.use_count()   1. ,   get()   -  static_cast<T*>(p) . 
 
   :  other   shared_ptr , ,    ,   shared_ptr   ,    use_count() == 0 .      ,  p    NULL. 
 
 int i; shared_ptr<int> sp(shared_ptr<X>(),&i); assert(sp.use_count()==0); assert(sp.get()==&i); 
          . 
 
     
            shared_ptr ,                . 
 
 ,     ,          . 
 
      reddit 
    cppreference 

shared_ptr, ( - ).

, , shared_ptr' . , , , .

, ? shared_ptr , ( , ) .


, X, Y,

struct X{ Y y; };
, shared_ptr, px px->y , shared_ptr . : shared_ptr, , , , Y. , , shared_ptr , px ?

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); } // X
shared_ptr , , , . , . , , , shared_ptr shared_ptr. shared_ptr .

void bar(){ std::shared_ptr<X> px(std::make_shared<X>()); std::shared_ptr<Y> py(px,&px->y); store_for_later(py); } // X
. - , shared_ptr . X2, Y,

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); } // X2
, pimpl . , , , . . X , shared_ptr, shared_ptr , shared_ptr (.. reset() ).


:

template<typename Other,typename Target> shared_ptr(shared_ptr<Other> const& other,Target* p);
shared_ptr p * , . Other . other , other.use_count() 1. , get() - static_cast<T*>(p) .

: other shared_ptr , , , shared_ptr , use_count() == 0 . , p NULL.

int i; shared_ptr<int> sp(shared_ptr<X>(),&i); assert(sp.use_count()==0); assert(sp.get()==&i);
.


shared_ptr , .

, , .

reddit
cppreference

shared_ptr, ( - ).

, , shared_ptr' . , , , .

, ? shared_ptr , ( , ) .


, X, Y,

struct X{ Y y; };
, shared_ptr, px px->y , shared_ptr . : shared_ptr, , , , Y. , , shared_ptr , px ?

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); } // X
shared_ptr , , , . , . , , , shared_ptr shared_ptr. shared_ptr .

void bar(){ std::shared_ptr<X> px(std::make_shared<X>()); std::shared_ptr<Y> py(px,&px->y); store_for_later(py); } // X
. - , shared_ptr . X2, Y,

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); } // X2
, pimpl . , , , . . X , shared_ptr, shared_ptr , shared_ptr (.. reset() ).


:

template<typename Other,typename Target> shared_ptr(shared_ptr<Other> const& other,Target* p);
shared_ptr p * , . Other . other , other.use_count() 1. , get() - static_cast<T*>(p) .

: other shared_ptr , , , shared_ptr , use_count() == 0 . , p NULL.

int i; shared_ptr<int> sp(shared_ptr<X>(),&i); assert(sp.use_count()==0); assert(sp.get()==&i);
.


shared_ptr , .

, , .

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cppreference

shared_ptr, ( - ).

, , shared_ptr' . , , , .

, ? shared_ptr , ( , ) .


, X, Y,

struct X{ Y y; };
, shared_ptr, px px->y , shared_ptr . : shared_ptr, , , , Y. , , shared_ptr , px ?

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); } // X
shared_ptr , , , . , . , , , shared_ptr shared_ptr. shared_ptr .

void bar(){ std::shared_ptr<X> px(std::make_shared<X>()); std::shared_ptr<Y> py(px,&px->y); store_for_later(py); } // X
. - , shared_ptr . X2, Y,

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); } // X2
, pimpl . , , , . . X , shared_ptr, shared_ptr , shared_ptr (.. reset() ).


:

template<typename Other,typename Target> shared_ptr(shared_ptr<Other> const& other,Target* p);
shared_ptr p * , . Other . other , other.use_count() 1. , get() - static_cast<T*>(p) .

: other shared_ptr , , , shared_ptr , use_count() == 0 . , p NULL.

int i; shared_ptr<int> sp(shared_ptr<X>(),&i); assert(sp.use_count()==0); assert(sp.get()==&i);
.


shared_ptr , .

, , .

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cppreference

 shared_ptr,        (    -     ). 
 
 ,     ,     shared_ptr'   .       ,   ,  ,      . 
 
        ,      ?     shared_ptr ,     ( , )       . 
 
    
 ,    X,    Y,      
 
 struct X{ Y y; }; 
  ,      shared_ptr, px            px->y ,    shared_ptr .     :  shared_ptr,      ,    ,    ,        Y.     ,    ,  shared_ptr   ,  px    ? 
 
 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); } //  X  
  shared_ptr     , ,  , .    ,    .  ,    ,    ,     shared_ptr  shared_ptr.  shared_ptr    . 
 
 void bar(){ std::shared_ptr<X> px(std::make_shared<X>()); std::shared_ptr<Y> py(px,&px->y); store_for_later(py); } //  X    
       .   -          ,     shared_ptr .       X2,     Y,         
 
 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); } //  X2    
       ,    pimpl .   ,         ,   ,    .              .    X        ,     shared_ptr,     shared_ptr      ,  shared_ptr      (..   reset() ). 
 
  
  : 
 
 template<typename Other,typename Target> shared_ptr(shared_ptr<Other> const& other,Target* p); 
     shared_ptr  p    * ,    .    Other   .       other ,   other.use_count()   1. ,   get()   -  static_cast<T*>(p) . 
 
   :  other   shared_ptr , ,    ,   shared_ptr   ,    use_count() == 0 .      ,  p    NULL. 
 
 int i; shared_ptr<int> sp(shared_ptr<X>(),&i); assert(sp.use_count()==0); assert(sp.get()==&i); 
          . 
 
     
            shared_ptr ,                . 
 
 ,     ,          . 
 
      reddit 
    cppreference 

shared_ptr, ( - ).

, , shared_ptr' . , , , .

, ? shared_ptr , ( , ) .


, X, Y,

struct X{ Y y; };
, shared_ptr, px px->y , shared_ptr . : shared_ptr, , , , Y. , , shared_ptr , px ?

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); } // X
shared_ptr , , , . , . , , , shared_ptr shared_ptr. shared_ptr .

void bar(){ std::shared_ptr<X> px(std::make_shared<X>()); std::shared_ptr<Y> py(px,&px->y); store_for_later(py); } // X
. - , shared_ptr . X2, Y,

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); } // X2
, pimpl . , , , . . X , shared_ptr, shared_ptr , shared_ptr (.. reset() ).


:

template<typename Other,typename Target> shared_ptr(shared_ptr<Other> const& other,Target* p);
shared_ptr p * , . Other . other , other.use_count() 1. , get() - static_cast<T*>(p) .

: other shared_ptr , , , shared_ptr , use_count() == 0 . , p NULL.

int i; shared_ptr<int> sp(shared_ptr<X>(),&i); assert(sp.use_count()==0); assert(sp.get()==&i);
.


shared_ptr , .

, , .

reddit
cppreference

 shared_ptr,        (    -     ). 
 
 ,     ,     shared_ptr'   .       ,   ,  ,      . 
 
        ,      ?     shared_ptr ,     ( , )       . 
 
    
 ,    X,    Y,      
 
 struct X{ Y y; }; 
  ,      shared_ptr, px            px->y ,    shared_ptr .     :  shared_ptr,      ,    ,    ,        Y.     ,    ,  shared_ptr   ,  px    ? 
 
 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); } //  X  
  shared_ptr     , ,  , .    ,    .  ,    ,    ,     shared_ptr  shared_ptr.  shared_ptr    . 
 
 void bar(){ std::shared_ptr<X> px(std::make_shared<X>()); std::shared_ptr<Y> py(px,&px->y); store_for_later(py); } //  X    
       .   -          ,     shared_ptr .       X2,     Y,         
 
 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); } //  X2    
       ,    pimpl .   ,         ,   ,    .              .    X        ,     shared_ptr,     shared_ptr      ,  shared_ptr      (..   reset() ). 
 
  
  : 
 
 template<typename Other,typename Target> shared_ptr(shared_ptr<Other> const& other,Target* p); 
     shared_ptr  p    * ,    .    Other   .       other ,   other.use_count()   1. ,   get()   -  static_cast<T*>(p) . 
 
   :  other   shared_ptr , ,    ,   shared_ptr   ,    use_count() == 0 .      ,  p    NULL. 
 
 int i; shared_ptr<int> sp(shared_ptr<X>(),&i); assert(sp.use_count()==0); assert(sp.get()==&i); 
          . 
 
     
            shared_ptr ,                . 
 
 ,     ,          . 
 
      reddit 
    cppreference 

shared_ptr, ( - ).

, , shared_ptr' . , , , .

, ? shared_ptr , ( , ) .


, X, Y,

struct X{ Y y; };
, shared_ptr, px px->y , shared_ptr . : shared_ptr, , , , Y. , , shared_ptr , px ?

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); } // X
shared_ptr , , , . , . , , , shared_ptr shared_ptr. shared_ptr .

void bar(){ std::shared_ptr<X> px(std::make_shared<X>()); std::shared_ptr<Y> py(px,&px->y); store_for_later(py); } // X
. - , shared_ptr . X2, Y,

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); } // X2
, pimpl . , , , . . X , shared_ptr, shared_ptr , shared_ptr (.. reset() ).


:

template<typename Other,typename Target> shared_ptr(shared_ptr<Other> const& other,Target* p);
shared_ptr p * , . Other . other , other.use_count() 1. , get() - static_cast<T*>(p) .

: other shared_ptr , , , shared_ptr , use_count() == 0 . , p NULL.

int i; shared_ptr<int> sp(shared_ptr<X>(),&i); assert(sp.use_count()==0); assert(sp.get()==&i);
.


shared_ptr , .

, , .

reddit
cppreference

shared_ptr, ( - ).

, , shared_ptr' . , , , .

, ? shared_ptr , ( , ) .


, X, Y,

struct X{ Y y; };
, shared_ptr, px px->y , shared_ptr . : shared_ptr, , , , Y. , , shared_ptr , px ?

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); } // X
shared_ptr , , , . , . , , , shared_ptr shared_ptr. shared_ptr .

void bar(){ std::shared_ptr<X> px(std::make_shared<X>()); std::shared_ptr<Y> py(px,&px->y); store_for_later(py); } // X
. - , shared_ptr . X2, Y,

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); } // X2
, pimpl . , , , . . X , shared_ptr, shared_ptr , shared_ptr (.. reset() ).


:

template<typename Other,typename Target> shared_ptr(shared_ptr<Other> const& other,Target* p);
shared_ptr p * , . Other . other , other.use_count() 1. , get() - static_cast<T*>(p) .

: other shared_ptr , , , shared_ptr , use_count() == 0 . , p NULL.

int i; shared_ptr<int> sp(shared_ptr<X>(),&i); assert(sp.use_count()==0); assert(sp.get()==&i);
.


shared_ptr , .

, , .

reddit
cppreference

shared_ptr, ( - ).

, , shared_ptr' . , , , .

, ? shared_ptr , ( , ) .


, X, Y,

struct X{ Y y; };
, shared_ptr, px px->y , shared_ptr . : shared_ptr, , , , Y. , , shared_ptr , px ?

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); } // X
shared_ptr , , , . , . , , , shared_ptr shared_ptr. shared_ptr .

void bar(){ std::shared_ptr<X> px(std::make_shared<X>()); std::shared_ptr<Y> py(px,&px->y); store_for_later(py); } // X
. - , shared_ptr . X2, Y,

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); } // X2
, pimpl . , , , . . X , shared_ptr, shared_ptr , shared_ptr (.. reset() ).


:

template<typename Other,typename Target> shared_ptr(shared_ptr<Other> const& other,Target* p);
shared_ptr p * , . Other . other , other.use_count() 1. , get() - static_cast<T*>(p) .

: other shared_ptr , , , shared_ptr , use_count() == 0 . , p NULL.

int i; shared_ptr<int> sp(shared_ptr<X>(),&i); assert(sp.use_count()==0); assert(sp.get()==&i);
.


shared_ptr , .

, , .

reddit
cppreference

shared_ptr, ( - ).

, , shared_ptr' . , , , .

, ? shared_ptr , ( , ) .


, X, Y,

struct X{ Y y; };
, shared_ptr, px px->y , shared_ptr . : shared_ptr, , , , Y. , , shared_ptr , px ?

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); } // X
shared_ptr , , , . , . , , , shared_ptr shared_ptr. shared_ptr .

void bar(){ std::shared_ptr<X> px(std::make_shared<X>()); std::shared_ptr<Y> py(px,&px->y); store_for_later(py); } // X
. - , shared_ptr . X2, Y,

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); } // X2
, pimpl . , , , . . X , shared_ptr, shared_ptr , shared_ptr (.. reset() ).


:

template<typename Other,typename Target> shared_ptr(shared_ptr<Other> const& other,Target* p);
shared_ptr p * , . Other . other , other.use_count() 1. , get() - static_cast<T*>(p) .

: other shared_ptr , , , shared_ptr , use_count() == 0 . , p NULL.

int i; shared_ptr<int> sp(shared_ptr<X>(),&i); assert(sp.use_count()==0); assert(sp.get()==&i);
.


shared_ptr , .

, , .

reddit
cppreference

shared_ptr, ( - ).

, , shared_ptr' . , , , .

, ? shared_ptr , ( , ) .


, X, Y,

struct X{ Y y; };
, shared_ptr, px px->y , shared_ptr . : shared_ptr, , , , Y. , , shared_ptr , px ?

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); } // X
shared_ptr , , , . , . , , , shared_ptr shared_ptr. shared_ptr .

void bar(){ std::shared_ptr<X> px(std::make_shared<X>()); std::shared_ptr<Y> py(px,&px->y); store_for_later(py); } // X
. - , shared_ptr . X2, Y,

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); } // X2
, pimpl . , , , . . X , shared_ptr, shared_ptr , shared_ptr (.. reset() ).


:

template<typename Other,typename Target> shared_ptr(shared_ptr<Other> const& other,Target* p);
shared_ptr p * , . Other . other , other.use_count() 1. , get() - static_cast<T*>(p) .

: other shared_ptr , , , shared_ptr , use_count() == 0 . , p NULL.

int i; shared_ptr<int> sp(shared_ptr<X>(),&i); assert(sp.use_count()==0); assert(sp.get()==&i);
.


shared_ptr , .

, , .

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cppreference