- Qt 6.9
- Qt Core
- C++ Classes
- totally_ordered_wrapper
- Qt 6.9.3 Reference Documentation

# Qt::totally_ordered_wrapper Class

template <typename P> class Qt::totally_ordered_wrapper

Qt::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.