将现有的shared_ptr追加到shared_ptr的向量

Question

I have an existing vector of shared_ptr. I want to search through that vector and if a condition is met, copy the respective shared_ptr to a new vector.

...
//vector< shared_ptr<Foo> > main_vec; // which already has some data
vector< shared_ptr<Foo> > output_vec{};

for ( auto iter = main_vec.begin() ; iter != main_vec.end() ; ++iter )
{
  if ( (*iter)->bar() == true )
    output_vec.push_back( *iter );
}

return output_vec;
...

I am not convinced the above is correct?? I am guessing that this will copy the shared_ptr but not increase the ref_count of the original, or am I over thinking this? I'm pretty new to smart pointer.

TIA

Answer 1

I am not convinced the above is correct??

In accordance with your requirement specification, that's correct.

---

If you want to know more...

In particular, the statement:

output_vec.push_back( *iter );

has the following effects:

• *iter returns a reference to the smart pointer, ie, std::shared_ptr<Foo>& .
• output_vec.push_back will create a new smart pointer, invoking the copy-constructor .
• The copy constructor of std::shared_ptr :

  Constructs a shared_ptr which shares ownership of the object managed by r.

So the reference counter to the shared object will be increased.

Additional Notes...

Just for the sake of completeness, I would add some personal suggestions.

1) For-each loop can be expressed in a better way:

for (const auto& ptr : main_vec) {
  if (ptr->bar()) output_vec(ptr);
}

2) In particular, this for-each can be synthesized with copy_if .

Answer 2

I am guessing that this will copy the shared_ptr

Correct.

but not increase the ref_count of the original

No, the copy constructor does increase the reference count. That's how shared pointers are designed to be copyable.

I am not convinced the above is correct?

Then let me convince you: It is correct.