C ++ STL-STL sort()的第三个参数如何工作? [英] C++ STL - How does the third argument of the STL sort() work?
问题描述
我希望根据数据对象'age'对class Person的对象数组进行排序.我将对象存储在vector<Person> v.
I wish to sort an array of objects of class Person on the basis of its data member 'age'. I store the objects in a vector<Person> v.
据我所知，至少有4种方法可以执行此操作，并且基于以下编写的方法，我有以下问题.
As far I have understood, there can be at least 4 ways to perform this action and I have the following questions based on the methods written below.
-
在类内部定义的
operator()如何工作?我是否不应该使'<'重载运营商在这里也是如此?为什么是'()'?
How does
operator()defined inside a class work? Should I not overload the '<' operator here as well? Why '()' ?
我在方法1中发送了一个对象作为第三个参数.但是，在方法2中，我发送了函数的名称.为什么会这样?
I sent an object as the 3rd parameter in method 1. But, in method 2, I sent the name of a function. Why is it like that?
这四种方法中哪一种是最好的?我觉得方法3最简单.
Which of the four methods is the best? I felt that method 3 was the easiest.
方法1
class cmp
{
public:
bool operator() ( Person const &a, Person const &b )
{
return a.age < b.age ;
}
};
sort( v.begin(), v.end(), cmp());
方法2
bool cmp( const Person a, const Person b )
{
return a.age < b.age ;
}
sort( v.begin(), v.end(), cmp );
方法3
bool operator < ( const Person a, const Person b )
{
return a.age < b.age ;
}
sort( v.begin(), v.end());
方法4
//using lambda expression
sort( v.begin(), v.end(), [](const Person &a, const Person &b){return a.age < b.age;});
推荐答案
要使用std::sort(或与此相关的任何函数)对范围进行排序，它需要知道如何比较范围中的两个元素/em>，以确定小于(或大于)的关系.
To sort a range using std::sort (or any function for that matter), it needs to know how two elements from the range are compared, in order to determine less than (or greater than) relationship.
标准库函数std::sort有两种风格:一种使用operator<，另一种使用 compare 函数/功能.您已经在代码中同时使用了这两种方法—特别是，示例中的第三个使用<，其余的使用 compare function/functor.
The Standard Library function std::sort comes in two flavors: one uses operator<, the other uses a compare function/functor. You've used both of them in your code — in particular, the third one in your example uses < and the rest use compare function/functor.
关于哪种方法最好?
嗯，这取决于.使用operator<的代码是较不灵活的，因为它是固定的，但也要求您键入更少.足够时使用它.
Well, it depends. The one which uses operator< is less flexible since it is fixed but requires you less typing as well. Use it when it suffices.
另一个更灵活，因为您可以传递任何比较函数并相应地对元素进行排序.当operator<不足够时使用它.另外，当您选择这种口味时，您还有其他选择:比较器可以是 function ， functor 或 lambda &mdash ;如果使用函数或函子(在名称空间级别定义)，则可以重用；另一方面，lambda通常是在函数范围内定义的，因此，它不是可重用的，除非您在名称空间范围内定义它，在这种情况下，它几乎是相同的.作为功能.
The other one is more flexible as you can pass any compare function and get your elements sorted accordingly. Use it when operator< doesn't suffice. Also, when you choose this flavor, you have other choices as well : the comparer could be a function, a functor, or a lambda — if you use function or functor (defined at namespace level), then you can reuse them; on the other hand, lambda is usually defined in a function scope, so it is not that reusable, unless you define it at namespace scope, in which case it is almost same as function.
示例，假设您要对int的向量进行升序排序:
Example, suppose you want to sort a vector of int in increasing order:
std::vector<int> v{10, 3, 12, -26};
std::sort(v.begin(), v.end());
print(v);
输出:-26,3,10,12.因此operator<确实可以完成这项工作.
Output: -26,3,10,12. So operator< does do the job.
但是，如果您希望仅考虑幅值(即忽略符号)对元素进行排序，那您就必须使用另一种风味:
But what if you want the elements sorted considering only magnitude (i.e ignore signs), then you have to use the other flavor:
std::vector<int> v{10, 3, 12, -26};
auto abs_cmp = [](int a, int b) { return std::abs(a) < std::abs(b); };
std::sort(v.begin(), v.end(), abs_cmp);
print(v);
输出:3,10,12,-26.在这种情况下，这就是您期望的输出.
Output : 3,10,12,-26. That is the output you would expect in this case.
希望有帮助.
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