Qt::totally_ordered_wrapper Class

Detailed Description

One of its primary usecases is to prevent Undefined Behavior (UB) when comparing pointers.

Consider the following simple class:

template <typename T>
struct PointerWrapperBad {
    int val;
    T *ptr;
};

Lexicographical comparison of the two instances of the PointerWrapperBad type would result in UB, because it will call operator<() or operator<=>() on the ptr members.

To fix it, use the new wrapper type:

template <typename T>
struct PointerWrapperGood {
    int val;
    Qt::totally_ordered_wrapper<T *> ptr;

    friend bool
    operator==(PointerWrapperGood lhs, PointerWrapperGood rhs) noexcept = default;
    friend auto
    operator<=>(PointerWrapperGood lhs, PointerWrapperGood rhs) noexecpt = default;
};

The operator<() and (if available) operator<=>() operators for the Qt::totally_ordered_wrapper type use the std::less and std::compare_three_way function objects respectively, providing strict total order over pointers when doing the comparison.

As a result, the relational operators for PointerWrapperGood::ptr member will be well-defined, and we can even =default the relational operators for the PointerWrapperGood class, like it's shown above.