尝试将 std::pair 插入 std::set

我无法理解这段代码中的错误是什么：

#include <set>
#include <utility>
#include <iostream>

using namespace std;

class A
{
    public:
        A(unsigned int a) : _a(a) { }
        A() : _a(0) { }
        unsigned int a() const { return _a; }
    private:
        unsigned int _a;
};

class B
{
    public:
        B(unsigned int b) : _b(b) { }
        B() : _b(0) { }
        unsigned int b() const { return _b; }
    private:
        unsigned int _b;
};

void display(const Point& point)
{
    //cout << "A: " << point.first.a() << ", B: " << point.second.b() << endl;
}

typedef pair <A, B> Point;
typedef set <Point> List;

main()
{
    A a(5);
    B b(9);

    List list;
    List::iterator it;
    Point point;

    point = make_pair(a, b);

    it = list.begin();

    list.insert(point); // <--- error here

    //display(point);
}

错误是这样的：

In file included from /usr/lib/gcc/x86_64-pc-linux-gnu/4.4.4/include/g++-v4/bits/stl_algobase.h:66,
                 from /usr/lib/gcc/x86_64-pc-linux-gnu/4.4.4/include/g++-v4/bits/stl_tree.h:62,
                 from /usr/lib/gcc/x86_64-pc-linux-gnu/4.4.4/include/g++-v4/set:60,
                 from test.cpp:1:
/usr/lib/gcc/x86_64-pc-linux-gnu/4.4.4/include/g++-v4/bits/stl_pair.h: In function ‘bool std::operator<(const std::pair<_T1, _T2>&, const std::pair<_T1, _T2>&) [with _T1 = A, _T2 = B]’:
/usr/lib/gcc/x86_64-pc-linux-gnu/4.4.4/include/g++-v4/bits/stl_function.h:230:   instantiated from ‘bool std::less<_Tp>::operator()(const _Tp&, const _Tp&) const [with _Tp = std::pair<A, B>]’
/usr/lib/gcc/x86_64-pc-linux-gnu/4.4.4/include/g++-v4/bits/stl_tree.h:1170:   instantiated from ‘std::pair<typename std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::iterator, bool> std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_M_insert_unique(const _Val&) [with _Key = std::pair<A, B>, _Val = std::pair<A, B>, _KeyOfValue = std::_Identity<std::pair<A, B> >, _Compare = std::less<std::pair<A, B> >, _Alloc = std::allocator<std::pair<A, B> >]’
/usr/lib/gcc/x86_64-pc-linux-gnu/4.4.4/include/g++-v4/bits/stl_set.h:411:   instantiated from ‘std::pair<typename std::_Rb_tree<_Key, _Key, std::_Identity<_Key>, _Compare, typename _Alloc::rebind<_Key>::other>::const_iterator, bool> std::set<_Key, _Compare, _Alloc>::insert(const _Key&) [with _Key = std::pair<A, B>, _Compare = std::less<std::pair<A, B> >, _Alloc = std::allocator<std::pair<A, B> >]’
test.cpp:48:   instantiated from here
/usr/lib/gcc/x86_64-pc-linux-gnu/4.4.4/include/g++-v4/bits/stl_pair.h:154: error: no match for ‘operator<’ in ‘__x->std::pair<A, B>::second < __y->std::pair<A, B>::second’
/usr/lib/gcc/x86_64-pc-linux-gnu/4.4.4/include/g++-v4/bits/stl_pair.h:154: error: no match for ‘operator<’ in ‘__y->std::pair<A, B>::first < __x->std::pair<A, B>::first’
/usr/lib/gcc/x86_64-pc-linux-gnu/4.4.4/include/g++-v4/bits/stl_pair.h:154: error: no match for ‘operator<’ in ‘__x->std::pair<A, B>::first < __y->std::pair<A, B>::first’

You are trying to use std::set with an element type that does not have ordering (std::pair), while a set needs that its elements have "a specific strict weak ordering criterion".

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