12.1.6. weak_ptr

A weak_ptr (Table 12.5) is a smart pointer that does not control the lifetime of the object to which it points. Instead, a weak_ptr points to an object that is managed by a shared_ptr. Binding a weak_ptr to a shared_ptr does not change the reference count of that shared_ptr. Once the last shared_ptr pointing to the object goes away, the object itself will be deleted. That object will be deleted even if there are weak_ptrs pointing to it—hence the name weak_ptr, which captures the idea that a weak_ptr shares its object “weakly.”

Table 12.5. weak_ptrs

When we create a weak_ptr, we initialize it from a shared_ptr:

auto p = make_shared<int>(42);
weak_ptr<int> wp(p);  // wp weakly shares with p; use count in p is unchanged

Here both wp and p point to the same object. Because the sharing is weak, creating wp doesn’t change the reference count of p; it is possible that the object to which wp points might be deleted.

Because the object might no longer exist, we cannot use a weak_ptr to access its object directly. To access that object, we must call lock. The lock function checks whether the object to which the weak_ptr points still exists. If so, lock returns a shared_ptr to the shared object. As with any other shared_ptr, we are guaranteed that the underlying object to which that shared_ptr points continues to exist at least as long as that shared_ptr exists. For example:

if (shared_ptr<int> np = wp.lock()) { // true if np is not null
    // inside the if, np shares its object with p
}

Here we enter the body of the if only if the call to lock succeeds. Inside the if, it is safe to use np to access that object.

Checked Pointer Class

As an illustration of when a weak_ptr is useful, we’ll define a companion pointer class for our StrBlob class. Our pointer class, which we’ll name StrBlobPtr, will store a weak_ptr to the data member of the StrBlob from which it was initialized. By using a weak_ptr, we don’t affect the lifetime of the vector to which a given StrBlob points. However, we can prevent the user from attempting to access a vector that no longer exists.

StrBlobPtr will have two data members: wptr, which is either null or points to a vector in a StrBlob; and curr, which is the index of the element that this object currently denotes. Like its companion StrBlob class, our pointer class has a check member to verify that it is safe to dereference the StrBlobPtr:

// StrBlobPtr throws an exception on attempts to access a nonexistent element
class StrBlobPtr {
public:
    StrBlobPtr(): curr(0) { }
    StrBlobPtr(StrBlob &a, size_t sz = 0):
            wptr(a.data), curr(sz) { }
    std::string& deref() const;
    StrBlobPtr& incr();       // prefix version
private:
    // check returns a shared_ptr to the vector if the check succeeds
    std::shared_ptr<std::vector<std::string>>
        check(std::size_t, const std::string&) const;
    // store a weak_ptr, which means the underlying vector might be destroyed
    std::weak_ptr<std::vector<std::string>> wptr;
    std::size_t curr;      // current position within the array
};

The default constructor generates a null StrBlobPtr. Its constructor initializer list (§ 7.1.4, p. 265) explicitly initializes curr to zero and implicitly initializes wptr as a null weak_ptr. The second constructor takes a reference to StrBlob and an optional index value. This constructor initializes wptr to point to the vector in the shared_ptr of the given StrBlob object and initializes curr to the value of sz. We use a default argument (§ 6.5.1, p. 236) to initialize curr to denote the first element by default. As we’ll see, the sz parameter will be used by the end member of StrBlob.

It is worth noting that we cannot bind a StrBlobPtr to a const StrBlob object. This restriction follows from the fact that the constructor takes a reference to a nonconst object of type StrBlob.

The check member of StrBlobPtr differs from the one in StrBlob because it must check whether the vector to which it points is still around:

std::shared_ptr<std::vector<std::string>>
StrBlobPtr::check(std::size_t i, const std::string &msg) const
{
    auto ret = wptr.lock();   // is the vector still around?
    if (!ret)
        throw std::runtime_error("unbound StrBlobPtr");
    if (i >= ret->size())
        throw std::out_of_range(msg);
    return ret; // otherwise, return a shared_ptr to the vector
}

Because a weak_ptr does not participate in the reference count of its corresponding shared_ptr, the vector to which this StrBlobPtr points might have been deleted. If the vector is gone, lock will return a null pointer. In this case, any reference to the vector will fail, so we throw an exception. Otherwise, check verifies its given index. If that value is okay, check returns the shared_ptr it obtained from lock.

Pointer Operations

We’ll learn how to define our own operators in Chapter 14. For now, we’ve defined functions named deref and incr to dereference and increment the StrBlobPtr, respectively. The deref member calls check to verify that it is safe to use the vector and that curr is in range:

std::string& StrBlobPtr::deref() const
{
    auto p = check(curr, "dereference past end");
    return (*p)[curr];  // (*p) is the vector to which this object points
}

If check succeeds, p is a shared_ptr to the vector to which this StrBlobPtr points. The expression (*p)[curr] dereferences that shared_ptr to get the vector and uses the subscript operator to fetch and return the element at curr.

The incr member also calls check:

// prefix: return a reference to the incremented object
StrBlobPtr& StrBlobPtr::incr()
{
    // if curr already points past the end of the container, can't increment it
    check(curr, "increment past end of StrBlobPtr");
    ++curr;       // advance the current state
    return *this;
}

We’ll also give our StrBlob class begin and end operations. These members will return StrBlobPtrs pointing to the first or one past the last element in the StrBlob itself. In addition, because StrBlobPtr accesses the data member of StrBlob, wemust alsomake StrBlobPtr a friend of StrBlob (§ 7.3.4, p. 279):

class StrBlob {
    friend class StrBlobPtr;
    // other members as in § 12.1.1 (p. 456)
    StrBlobPtr begin(); // return StrBlobPtr to the first element
    StrBlobPtr end(); // and one past the last element
};
// these members can’t be defined until StrStrBlob and StrStrBlobPtr are defined
StrBlobPtr StrBlob::begin() { return StrBlobPtr(*this); }
StrBlobPtr StrBlob::end()
    { return StrBlobPtr(*this, data->size()); }