将 unordered_map 分配给一对对象 [英] Assigning of unordered_map to pair of objects

问题描述

我正在尝试理解 unordered_map 赋值，我收到以下错误:没有匹配的函数用于调用 std::pair<foo, foo>::pair()，根据 unordered_map operator[] 的文档:

I am trying to understand the unordered_map assignment, I get the following error: no matching function for call to std::pair<foo, foo>::pair(), according to the doc for unordered_map operator[]:

如果 k 与容器中任何元素的键都不匹配，该函数将插入一个具有该键的新元素并返回对其映射值的引用.

If k does not match the key of any element in the container, the function inserts a new element with that key and returns a reference to its mapped value.

所以我试图将一个对象(来自 make_pair)分配给这个引用，我猜这是不允许的.但是，对于 pair，它可以工作，然后我想知道是否必须为 foo 声明一些其他运算符才能使其工作.

So I am trying to assign an object (from make_pair) to this reference, which I am guessing is not allowed. However with the pair<int,int>, it works, then I am wondering if I must declare some other operators for foo to make this work.

#include <bits/stdc++.h>
using namespace std;

struct foo {
  int n;
  foo(int n): n(n) {};
};

int main(){
  unordered_map<int, pair<foo,foo>> m;
  //m[3] = make_pair(foo(1),foo(2));         <--- error here

  unordered_map<int, pair<int,int>> ii;
  ii[3] = make_pair(1,2);
}

推荐答案

问题在于 operator [] 可能必须构造一个值对象，在您的情况下为 std::pair<foo, foo>.由于 foo 没有默认构造函数，所以它不能构造默认的 std::pair.

The problem is that operator [] might have to construct a value object, in your case std::pair<foo, foo>. Since foo doesn't have a default constructor, it can't construct the default std::pair.

您可以为 foo 提供默认构造函数(包括为 n 添加默认值)，或者您必须使用另一种方法将值插入 <代码>m.

You can either provide a default constructor for foo (including adding a default value for n), or you'll have to use another method to insert values into m.

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