运算符重载中没有运算符'=='匹配[重复]

Question

这个问题在这里已有答案：

Operator Overloading in C++ as int + obj 4个答案

我正在研究“Roman to int”算法，我的代码如下。我收到一个错误：

no operator "==" matches these operands -- operand types are: char == const Solution::symbol.

有人可以帮我修复代码吗？

// solution.h
#include <string>
using namespace std;

class Solution {
  private:
    struct symbol {
      char upperCase;
      char lowerCase;
      bool operator ==(char ch) {
        return ch == upperCase || ch == lowerCase;
      };
    };
    static constexpr symbol one {'I', 'i'};
    static constexpr symbol five {'V', 'v'};
    static constexpr symbol ten {'X', 'x'};
    static constexpr symbol fifty {'L', 'l'};
    static constexpr symbol hundred {'C', 'c'};
    static constexpr symbol fiveHundred {'D', 'd'};
    static constexpr symbol thousand {'M', 'm'};
  public:
    bool romanToInt() {
      char ch = 'I';
      ch == one; // ERROR: no operator "==" matches these operands -- operand types a re: char == const Solution::symbol
      one == ch; // ERROR: no operator "==" matches these operands -- operand types a re: const Solution::symbol == char
    };
};

// main.cpp
#include <iostream>
#include "../Header Files/solution.h"
using namespace std;

int main() {
  Solution solution;
  solution.romanToInt();

  return 0;
}