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. 更多...

头：	#include <QtCompare>
CMake:	find_package(Qt6 REQUIRED COMPONENTS Core) target_link_libraries(mytarget PRIVATE Qt6::Core)
qmake:	QT += core
Since:	Qt 6.8

详细描述

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<=>() 在 ptr 成员。

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.

内容

1. 详细描述