试图将 std::pair 插入 std::set [英] trying to insert std::pair into std::set
问题描述
我无法理解这段代码中的错误是什么:
i can't understand what the error is in this code:
#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’
推荐答案
您正在尝试将 std::set
与没有排序的元素类型 (,而集合需要其元素具有特定的严格弱排序标准".std::pair
)
You are trying to use std::set
with an element type that does not have ordering (, while a set needs that its elements have "a specific strict weak ordering criterion".std::pair
)
更新:实际上std::pair
确实提供了一个operator<
(感谢 @UncleBens),这是根据其组件的 operator<
定义的;所以问题在于你的 A
和 B
没有提供比较运算符;你应该为 A
和 B
编写一个 operator<
.
Update: actually std::pair
does provide an operator<
(thanks @UncleBens), that is defined in terms of the operator<
of its components; so the problem lies in your A
and B
not providing a comparison operator; you should write an operator<
for A
and B
.
或者,由于 operator<
通常对点没有意义,您可以为 Point
创建一个比较函子并将其作为std::set
的第二个模板参数.
In alternative, since an operator<
in general doesn't really make sense for points, you can create a comparison functor for your Point
s and pass it as the second template argument for std::set
.
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