Returning the raw pointer from a list of unique pointers

Question

I have a class (A) that will own the memory for a set of instances of another class (B). A will store its instances of B as a set of std::unique_ptr<B>.

I want A to be able to return a reference to a set of raw pointers to those B objects, (i.e. I don't want any transfer of ownership). What is the best way to do this?

class A
{
private:
    std::set<std::unique_ptr<B>> _Bs;
public:
    std::set<B*>& getBs() { ???? };
}

I could create a new vector:

class A
{
private:
    std::set<std::unique_ptr<B>> _Bs;
    std::set<B*>                 _rawBs;
public:
    std::set<B*>& getBs() { return _rawBs; };
}

std::set<B*>
A::A()
{
    std::transform(_Bs.begin(), 
                   _Bs.end(), 
                   std::inserter(_rawBs, _rawBs.begin()), 
                   [](auto uniq_ptr) { uniq_ptr.get(); }
    
}

But it seems a bit janky to store these pointers twice...

How about returning cosnt std::set<std::unique_ptr<B>> &? This isn't an ownership transfer. — HolyBlackCat
– HolyBlackCat, Commented Apr 18, 2024 at 15:32
I'm surprised std::set of std::unique_ptr even works. What does the comparison operator do? — Mark Ransom
– Mark Ransom, Commented Apr 18, 2024 at 15:39
@MarkRansom Probably the same thing std::less does for raw pointers. — NathanOliver
– NathanOliver, Commented Apr 18, 2024 at 15:42
I would just return a const & for the existing set. If the user wants a copy, they can make one themselves. As a side note, I don't even understand the usefulness of a set here... — Christian Stieber
– Christian Stieber, Commented Apr 18, 2024 at 16:25
@MarkRansom en.cppreference.com/w/cpp/memory/unique_ptr/operator_cmp — Remy Lebeau
– Remy Lebeau, Commented Apr 18, 2024 at 17:51

Jan Schultke · Accepted Answer · 2024-04-19 12:03:05Z

You should add a member function:

const std::set<std::unique_ptr<B>>& getBs() {
    return m_bs;
}

There are a few things to note:

Unfortunately you cannot return something like std::span and have to return a reference to a const std::set. This leaks more implementation details, but is necessary because sets aren't contiguous containers.
You can still return a std::vector<B*> that contains a copy of all the pointers, but this would be more expensive.
A const std::set only lets you access the elements inside as const std::unique_ptr, so no ownership is being transferred, despite exposing the std::unique_ptr. Transfer of ownership would also require the ability to modify the std::unique_ptr.
You should not use the name _Bs for a data member because an underscore followed by a capital letter makes for a reserved identifier. See What are the rules about using an underscore in a C++ identifier?. The most popular naming scheme for members (if you give them a prefix or suffix) is the prefix m_.

Caleth · Accepted Answer · 2024-04-19 12:21:44Z

0

I want A to be able to return a reference to a set of raw pointers to those B objects

That's a bad plan. What do you do if a caller modifies that set?

You could instead return a view of pointers, by value.

class A
{
private:
    std::set<std::unique_ptr<B>> Bs;
public:
    auto getBs() const { 
        return Bs | std::views::transform([](auto & ptr){ return ptr.get(); }); 
    };
};

If your std::unique_ptr<B>s are guaranteed to be non-null, you could instead return a view of references by swapping the ptr.get() for *ptr.

answered Apr 19, 2024 at 12:21

Caleth

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