Qt::totally_ordered_wrapper< P > Class Template Reference

\inmodule QtCore \inheaderfile QtCompare More...

#include <qcomparehelpers.h>

Collaboration diagram for Qt::totally_ordered_wrapper< P >:

Public Member Functions
	totally_ordered_wrapper () noexcept=default
Q_IMPLICIT constexpr	totally_ordered_wrapper (std::nullptr_t)
constexpr	totally_ordered_wrapper (P p) noexcept
constexpr P	get () const noexcept
constexpr void	reset (P p) noexcept
constexpr P	operator-> () const noexcept
template<typename U = T, std::enable_if_t<!std::is_void_v< U >, bool > = true>
constexpr U &	operator* () const noexcept
constexpr	operator bool () const noexcept

Friends
void	qt_ptr_swap (totally_ordered_wrapper &lhs, totally_ordered_wrapper &rhs) noexcept
void	swap (totally_ordered_wrapper &lhs, totally_ordered_wrapper &rhs) noexcept
size_t	qHash (totally_ordered_wrapper key, size_t seed=0) noexcept

Detailed Description

template<typename P>
class Qt::totally_ordered_wrapper< P >

\inmodule QtCore \inheaderfile QtCompare

Qt::totally_ordered_wrapper is a wrapper type that provides strict total order for the wrapped types.

Since

6.8

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 \l {https://en.cppreference.com/w/cpp/utility/functional/less}{std::less} and \l {https://en.cppreference.com/w/cpp/utility/compare/compare_three_way} {std::compare_three_way} function objects respectively, providing \l {https://en.cppreference.com/w/cpp/language/operator_comparison#Pointer_total_order} {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.

Definition at line 1087 of file qcomparehelpers.h.

Constructor & Destructor Documentation

totally_ordered_wrapper() [1/3]

template<typename P>

Qt::totally_ordered_wrapper< P >::totally_ordered_wrapper ( )

defaultnoexcept

totally_ordered_wrapper() [2/3]

template<typename P>

Q_IMPLICIT constexpr Qt::totally_ordered_wrapper< P >::totally_ordered_wrapper ( std::nullptr_t )

inlineconstexpr

Definition at line 1095 of file qcomparehelpers.h.

totally_ordered_wrapper() [3/3]

template<typename P>

Qt::totally_ordered_wrapper< P >::totally_ordered_wrapper ( P p )

inlineexplicitconstexprnoexcept

Definition at line 1098 of file qcomparehelpers.h.

Member Function Documentation

get()

template<typename P>

P Qt::totally_ordered_wrapper< P >::get ( ) const

inlineconstexprnoexcept

Definition at line 1100 of file qcomparehelpers.h.

Referenced by operator bool(), operator*(), and operator->().

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operator bool()

template<typename P>

Qt::totally_ordered_wrapper< P >::operator bool ( ) const

inlineexplicitconstexprnoexcept

Definition at line 1106 of file qcomparehelpers.h.

References get().

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operator*()

template<typename P>

template<typename U = T, std::enable_if_t<!std::is_void_v< U >, bool > = true>

U & Qt::totally_ordered_wrapper< P >::operator* ( ) const

inlineconstexprnoexcept

Definition at line 1104 of file qcomparehelpers.h.

References get().

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operator->()

template<typename P>

P Qt::totally_ordered_wrapper< P >::operator-> ( ) const

inlineconstexprnoexcept

Definition at line 1102 of file qcomparehelpers.h.

References get().

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reset()

template<typename P>

void Qt::totally_ordered_wrapper< P >::reset ( P p )

inlineconstexprnoexcept

Definition at line 1101 of file qcomparehelpers.h.

Friends And Related Symbol Documentation

qHash

template<typename P>

size_t qHash ( totally_ordered_wrapper< P > key, size_t seed = 0 )

friend

Definition at line 1148 of file qcomparehelpers.h.

qt_ptr_swap

template<typename P>

void qt_ptr_swap ( totally_ordered_wrapper< P > & lhs, totally_ordered_wrapper< P > & rhs )

friend

Definition at line 1144 of file qcomparehelpers.h.

swap

template<typename P>

void swap ( totally_ordered_wrapper< P > & lhs, totally_ordered_wrapper< P > & rhs )

friend

Definition at line 1146 of file qcomparehelpers.h.

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The documentation for this class was generated from the following file:

• qtbase/src/corelib/global/qcomparehelpers.h (8784ea16a6bc66ac481d5cbf2dd1ece2d57a836b)