C ++:在多集容器中使用自己的类 [英] C++: use own class in a multiset container

问题描述

起初我是新来的，英语不是我的母语，所以对于任何语法上的错误，我深表歉意，但是我发现这个社区真的很好，所以我将尝试尽可能精确地提出我的问题.

at first I'm new here and English isn't my native language so apologize for any grammatical failures but I find this community really nice so I will try to ask my question as precise as I can.

我想将自己的类对象添加到stl容器多集中，并希望使用在类中定义的我自己的重载less运算符对它进行排序.我确实尝试了几种解决方案，但没有任何实际效果，因此希望有人能给我一些有用的提示来解决它.

I want to add my own class object into a stl container multiset and want to sort it with my own overloaded less operator defined in my class. I really tried out several solutions but nothing really worked so I hope someone can give me some useful hints to solve it.

这是我对班级定义的一般理解:

Here is my general idea of my class definition:

class object {
public:
    int first;
    string second;

    object(int f, string s) {
         first = f;
         second = s;
    }

    bool operator<(const object &comp) {
        return first < comp.first;
        }
};

这是我的第一次尝试，但是没有用，所以我也尝试将重载的运算符声明为朋友方法，但是它也没有用.

This was my first try and it didn't work so I also tried out to declare the overloaded operator as a friend method but it didn't work also.

这是我主要功能的简短代码摘录:

Here is a short code extract from my main function:

includes ...
//code omitted
int main() {
    multiset<object*> mmset;

    mmset.insert(new object(10, "test"));
    mmset.insert(new object(11, "test"));

    return 0;
 }

一段时间后，我开始调试代码，尝试找出问题出在哪里，然后发现以下使我有些怀疑的事情.

After a while I started to debugging my code and try to figure out where the problem is and I come across the following thing that have made me a bit suspicious.

从stl中提取代码:

// TEMPLATE STRUCT less
template<class _Ty>
struct less : public binary_function<_Ty, _Ty, bool>
    {   // functor for operator<
    bool operator()(const _Ty& _Left, const _Ty& _Right) const
    {   // apply operator< to operands
          return (_Left < _Right);
    }
    };

我已经在此行上设置了一个断点，并观察了程序在这里做什么，我不知道为什么，但是它仅比较两个对象的地址并始终返回false.尽管该运算符存在并且_Left和_Right变量包含我对象的地址，但它从未调用过重载的less运算符.

I have set a breakpoint on this line and observed what the program is doing here and I don't know why, but it only compares the addresses from the two objects and return so always false. It never calls my overloaded less operator although the operator exists and the _Left and _Right variables contain the address to my object.

如果有人能帮助我，我将非常感激.

I would really appreciate it if someone could help me.

最好的问候

汤姆

推荐答案

您没有在multiset中存储object.您正在存储object*.这些是指向object的指针.这意味着该集合将对要插入其中的指针进行排序.

You are not storing objects in your multiset. You are storing object*s. These are pointers to objects. This means the set will order the pointers that you're inserting into it.

似乎您真的只想要一个multiset<object>:

It seems like you really just want a multiset<object>:

multiset<object> mmset;
mmset.emplace(10, "test");
mmset.emplace(11, "test");

现在，它将使用<来比较object本身.

Now it will use < to compare the objects themselves.

如果您确实要存储指针，则需要为multiset提供一个自定义比较器.在C ++ 11中，您可以使用lambda轻松做到这一点:

If you really want to store pointers, you'll need to provide a custom comparator to the multiset. In C++11, you can do this easily with a lambda:

auto f = [](int* a, int* b) { return *a < *b; };
std::multiset<int*, decltype(f)> mmset(f);

在C ++ 11之前的版本中，您可以创建一个函数对象，该对象使用与此lambda函数相同的主体来实现operator().

Pre-C++11, you can create a function object that implements operator() with the same body as this lambda function.

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