c++ - Smart pointer: set by reference, reset, set null, null-check or reset-check -

for first time, i'm using smart pointers in c++. i've question std::shared_ptr:

1. set pointer reference:

   mytoy mytoy_1, mytoy_2; set_mytoy(mytoy_1, some_data); set_mytoy(mytoy_2, some_data);   shared_ptr<mytoy> ptr_mytoy(&mytoy_1); 

2. reset , new assignment:

   ptr_mytoy.reset(&mytoy_2); 

3. reset without assignment:

   ptr_mytoy.reset(); 

4. set null (?):

   ptr_mytoy(nullptr); 

are these examples right?

how can check if smart pointer "empty" (for instance, after .reset()) or if null?

are these examples right?

the first 2 wrong: try initialise , reset shared_ptr object, not pointer.

update: question has been changed initialise them pointers automatic variables. still wrong: shared_ptr want delete them, , it's error delete wasn't created new.

usually, object created using new, although it's better use make_shared create you:

// auto ptr = make_shared<mytoy>();  // not good, necessary mytoy * mytoy_2 = new mytoy; ptr.reset(mytoy_2); 

the third correct. release object pointer, deleting if there no remaining pointers, , leaving pointer empty.

the fourth either dubious or wrong, depending on null_ptr is. if mean nullptr, it's incorrect , shouldn't compile. if it's null-valued pointer mytoy, leaves shared_ptr non-empty, not owning either.

how can check if smart pointer "empty" (for instance, after .reset())

if (ptr.use_count() == 0) 

or if null?

if (!ptr)