关于c ++：使用对象插入密钥无法编译？

Insert with object as key fails to compile?

我找不到要编译的东西。我不明白被排除的错误由编译器编写。下面是一些说明问题的代码。

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#include <map>

using namespace std;

class Thing
{
public:
Thing(int n):val(n) {}

bool operator < (const Thing& rhs) const
{
return val < rhs.val;
}

int getVal() { return val; }

private:
int val;
};

int main(int argc, char* argv[])
{
std::map<Thing, int> mymap;
Thing t1(1);
Thing t2(10);
Thing t3(5);

mymap[t1] = 1; // OK

mymap.insert(t1); // Compile error
}

现在编译器错误消息：

test.cpp: In function ‘int main(int,
char**)’: test.cpp:34: error: no
matching function for call to
‘std::map,
std::allocator > >::insert(Thing&)’
/usr/include/c++/4.4/bits/stl_map.h:499:
note: candidates are:
std::pair,
std::_Select1st >, _Compare, typename _Alloc::rebind >::other>::iterator, bool> std::map<_Key, _Tp, _Compare, _Alloc>::insert(const std::pair&) [with _Key = Thing, _Tp = int, _Compare = std::less,
_Alloc = std::allocator >]
/usr/include/c++/4.4/bits/stl_map.h:539:
note: typename
std::_Rb_tree<_Key, std::pair, std::_Select1st >, _Compare, typename _Alloc::rebind >::other>::iterator std::map<_Key, _Tp, _Compare, _Alloc>::insert(typename std::_Rb_tree<_Key, std::pair, std::_Select1st >, _Compare, typename _Alloc::rebind >::other>::iterator, const std::pair&) [with
_Key = Thing, _Tp = int, _Compare = std::less, _Alloc =
std::allocator >]

那是什么意思？我需要在thing中定义另一个方法或运算符来编译它吗？