shared_ptr<> is to weak_ptr<> as unique_ptr<> is to... what?

In C++11, you can use a shared_ptr<> to establish an ownership relation with an object or variable and weak_ptr<> to safely reference that object in a non-owned way.

You can also use unique_ptr<> to establish an ownership relation with an object or variable. But what if other, non-owning objects want to also reference that object? weak_ptr<> isn't helpful in this case. Raw pointers are helpful but bring various downsides (e.g. they can be automatically initialized to nullptr but this is accomplished through techniques that are not consistent with the std::*_ptr<> types).

What is the equivalent of weak_ptr<> for non-owning references to objects owned via unique_ptr<>?

Here's a clarifying example that resembles something in a game I'm working on.

class World
{
public:

    Trebuchet* trebuchet() const { return m_trebuchet.get(); }

private:
    std::unique_ptr< Trebuchet > m_trebuchet;
};

class Victim
{
public:
    Victim( Trebuchet* theTrebuchet ) : m_trebuchet( theTrebuchet ) {}

    ~Victim()
    {
        delete m_trebuchet;     // Duh. Oops. Dumb error. Nice if the compiler helped prevent this.
    }

private:

    Trebuchet* m_trebuchet;    // Non-owning.
};

shared_ptr< Victim > createVictim( World& world )
{
    return make_shared< Victim >( world.trebuchet() );
}

Here we use a raw pointer to maintain a non-owning relationship with an object owned via unique_ptr<> elsewhere. But is raw the best we can do?

The hope is a type of pointer that:

Looks like the other modern pointer types. E.g. std::raw_ptr<T>.
Replaces raw pointers so that a codebase that uses modern pointer types throughout can find all pointers via a search for _ptr< (roughly).
Auto-initializes to nullptr.

Thus:

int* p;                  // Unknown value.
std::raw_ptr< int > p;   // null.

Does this type already exist in C++ now, is it proposed for the future, or is another implementation broadly available in e.g. Boost?

The "notify" behavior of shared_ptr requires reference counting the reference count control block. shared_ptr's reference count control block(s) use separate reference counts for this. weak_ptr instances maintain references to this block, and weak_ptrs themselves prevent the reference count control block from being deleteed. The pointed-to object has its destructor called when the strong count goes to zero (which may or may not result in deleteion of the memory where that object was stored), and the control block is deleteed only when the weak reference count goes to zero.

unique_ptr's tenet is that it has zero overhead over a plain pointer. Allocating and maintaining reference count control blocks (to support weak_ptr-ish semantics) breaks that tenet. If you need behavior of that description, then you really want shared semantics, even if other references to the object are non-owning. There's still sharing going on in that case -- the sharing of the state of whether or not the object has been destroyed.

If you need a generic nonowning reference and don't need notification, use plain pointers or plain references to the item in the unique_ptr.

EDIT:

In the case of your example, it looks like Victim should ask for a Trebuchet& rather than a Trebuchet*. Then it's clear who owns the object in question.

class World
{
public:

    Trebuchet& trebuchet() const { return *m_trebuchet.get(); }

private:
    std::unique_ptr< Trebuchet > m_trebuchet;
};

class Victim
{
public:
    Victim( Trebuchet& theTrebuchet ) : m_trebuchet( theTrebuchet ) {}

    ~Victim()
    {
        delete m_trebuchet;     // Compiler error. :)
    }

private:

    Trebuchet& m_trebuchet;    // Non-owning.
};

shared_ptr< Victim > createVictim( World& world )
{
    return make_shared< Victim >( world.trebuchet() );
}

There is a genuine need for a standard pointer type to act as a non-owning, inexpensive, and well-behaved counterpoint to std::unique_ptr<>. No such pointer has been standardized yet, but a standard has been proposed and is under discussion by the C++ standards committee. The "World's Dumbest Smart Pointer", aka std::exempt_ptr<> would have the general semantics of other modern C++ pointer classes but would hold no responsibility either for owning the pointed-to object (as shared_ptr and unique_ptr do) or for correctly responding to the deletion of that object (as weak_ptr does).

Assuming that this feature is ultimately ratified by the committee, it would fully meet the need highlighted in this question. Even if it isn't ratified by the committee, the above linked document fully expresses the need and describes a complete solution.

unique_ptr's non-owing analog is a plain C pointer. What is different - C pointer doesn't know if the pointed data is still accessible. weak_ptr on the other hand does. But it is impossible to replace raw pointer with a pointer knowing about the validity of data without additional overhead (and weak_ptr does have that overhead). That implies C-style pointer is the best in terms of speed you can get as a non-owing analog for unique_ptr.

shared_ptr<> is to weak_ptr<> as unique_ptr<> is to... what?

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