在C ++中使用自定义比较函数初始化多重集 [英] Initializing multiset with custom comparison function in C++

问题描述

考虑以下比较函数：

bool compare(std::shared_ptr<myObject> &lhs, std::shared_ptr<myObject> &rhs){
   return lhs->value < rhs->value;
}

现在的想法是初始化一个多集的类型 std :: shared_ptr< myObject> ，它使用上述函数对元素进行排序。所以从书我读的应该这样做：

Now idea is to initialize a multiset of type std::shared_ptr<myObject> which orders elements with above function. So from book i read it should be done like this:

std::multiset<std::shared_ptr<myObject>, decltype(compare)*> myset{compare};

问题：

我的问题是，在声明我undersad一个函数指针被传递来引用比较函数，但为什么我们initiazling set wtih {compare} ??它的重要性是什么，为什么有必要这样做呢？

My question is, in the declaration i understad a function pointer is passed to refer to compare function, but why are we initiazling the set wtih {compare}?? what is its importance and why is it necessary to do so like this??

推荐答案

因为集合需要一个比较函子与。如果你不指定一个，它将使用一个默认构造的。在这种情况下，由于你使用的是函数指针类型，默认构造的将是一个空指针，不能被调用;因此，您必须在运行时提供正确的函数指针。

Because the set needs a comparison functor to work with. If you don't specify one, it will make a default-constructed one. In this case, since you're using a function-pointer type, the default-constructed one will be a null pointer, which can't be called; so instead, you have to provide the correct function pointer at run time.

更好的方法是使用函数类类型（a.k.a. functor类型）。那么可以在编译时解析函数调用，并且默认构造的对象将做正确的事情：

A better approach might be to use a function class type (a.k.a. functor type); then the function call can be resolved at compile time, and a default-constructed object will do the right thing:

struct compare {
    bool operator()(std::shared_ptr<myObject> &lhs, 
                    std::shared_ptr<myObject> &rhs) const {
        return lhs->value < rhs->value;
    }
};

std::multiset<std::shared_ptr<myObject>, compare> myset;

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