Qt::totally_ordered_wrapper Class

template <typename P> class Qt::totally_ordered_wrapper

Qt::totally_ordered_wrapper 是一种封装类型，为封装类型提供严格的总顺序。更多

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

详细说明

它的主要用途之一是在比较指针时防止未定义行为(UB)。

请看下面这个简单的类：

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

对PointerWrapperBad 类型的两个实例进行词典比较会导致 UB，因为它会在ptr 成员上调用operator<() 或operator<=>() 。

要解决这个问题，可以使用新的封装类型：

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;
};

Qt::totally_ordered_wrapper 类型的operator<() 运算符和operator<=>() 运算符（如果可用）分别使用std::less和std::compare_three_way函数对象，在进行比较时提供严格的指针总顺序。

因此，PointerWrapperGood::ptr 成员的关系运算符将得到很好的定义，我们甚至可以=default PointerWrapperGood 类的关系运算符，如上图所示。

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