<<运算符使用迭代器作为参数 [英] Overloading of << operator using iterator as a parameter

问题描述

我喜欢将枚举值作为文本打印并用于重载。假设我有以下代码：

I`d like to print enum values as text and use for it overloading. Suppose I have the following code:

#include <iostream>
#include <map>
#include <string>
#include <vector>
#include <unordered_set>

enum enm{
    One,
    Two
};

class Complex{
public:
friend std::ostream& operator<<(std::ostream& out, std::unordered_multiset<int>::const_iterator i){
    switch (*i){
        case One:{
            return out<<"One";
        }
        case Two:{
            return out << "Two";
        }
    }
}
void func(std::unordered_multiset<int> _v);
};

void Complex:: func(std::unordered_multiset<int> _v){
    _v.insert(One);
    _v.insert(Two);
    for (std::unordered_multiset<int>::const_iterator i(_v.begin()), end(_v.end()); i != end; ++i){
        std::cout <<"Num: " << *i <<std::endl; //need to get here "One", "Two" instead of 0, 1
    }
}

int main(){
    Complex c;
    std::unordered_multiset<int> ms;
    c.func(ms);
    return 0;   
}

问题是这个变种doesn`t工作。所以，我得到0，1而不是一，二。没有想法如何正确地做。
谢谢您的帮助！

The problem is this variant doesn`t work. So, I get 0, 1 instead of One, Two. Have no ideas how to do it properly. Thank you for help!

推荐答案

我假设您将 i 更改为 * i ，以便于您的程序编译。为了打印迭代器，你必须做 i ，但这会失败，并出现编译错误。

I'm assuming you changed i to *i in order for your program to compile. In order to print the iterator you must do i, but this fails with a compiler error.

问题是插入运算符在它的第一个声明上被定义为类中的一个朋友，因此查找这个运算符只能依赖于与参数类型相关联的命名空间和类，一个称为 ADL 或Koenig查找。

The problem is that the insertion operator is defined as a friend within the class on its first declaration, so the lookup to find this operator can only depend on namespaces and classes associated with the argument types, a lookup referred to as ADL or Koenig lookup.

由于 std :: ostream 和 unordered_multiset :: const_iterator 不与 Complex （请参阅 ADL＃Details ），查找无法找到插入运算符。

Since std::ostream and unordered_multiset::const_iterator are not asscoiated with Complex (see ADL#Details), lookup fails to find the insertion operator.

在该类外部声明该函数，以便操作员正常无限制查找地点：

The solution is to declare the function outside of the class so that normal unqualified lookup for the operator takes place:

std::ostream& operator<<(std::ostream&, std::unordered_multiset<int>::const_iterator);
class Complex { .. };

但我建议您定义因为它似乎不需要访问 Complex （友好实体的目的的一部分）的私有/受保护成员。

I would recommend however that you define the operator outside of the class, as it doesn't seem to need to access private/protected members of Complex (part of the purpose of befriending entities).

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