Qt::totally_ordered_wrapper Class

Detailed Description

Qt::totally_ordered_wrapper<P> is a template class where P specifies the type to wrap.

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.