Qt::totally_ordered_wrapper Class
template <typename P> class Qt::totally_ordered_wrapperQt::totally_ordered_wrapper is a wrapper type that provides strict total order for the wrapped types. More...
| Header: | #include <QtCompare> |
| CMake: | find_package(Qt6 REQUIRED COMPONENTS Core)target_link_libraries(mytarget PRIVATE Qt6::Core) |
| qmake: | QT += core |
| Since: | Qt 6.8 |
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.