Reference Qualifiers for Class Member Functions

[robertefry]

Channel

This is a "C++Weekly" episode request

Topics

What's wrong with the following code?

class Keeper
{
    std::vector<int> m_Data {1,2,3};

public:
    auto& items() { return m_Data; }
};

Keeper GetKeeper()
{
    return {};
}

void UseKeeper()
{
    for (auto& item : GetKeeper().items())
    {
        std::cout << item << "\n";
    }
}

Keeper contains a vector of ints, which we construct in GetKeeper and use in UseKeeper. However, items() it returns a reference to the items, not the items themselves. So, when we call GetKeeper().items(), we then delete the underlying Keeper and it's items are left as a dangling reference. BAD!!!

To solve this, we can use reference qualifiers on items().

class Keeper
{
    std::vector<int> m_Data {1,2,3};

public:
    auto& items() & { return m_Data; }
    auto items() && { return m_Data; }
};

Now; auto& items() & is called on l-value Keepers, returning a reference to the underlying data; and auto items() && is called on r-value Keepers, returning (moving) the underlying data itself.

Length

I imagine this episode shouldn't take more than 10 minutes.

[robertefry]

The blogs below do nice write-ups of this issue, which I've copied my above example from.

https://andreasfertig.blog/2022/07/the-power-of-ref-qualifiers/ https://akrzemi1.wordpress.com/2014/06/02/ref-qualifiers/