No operator '==' matches in operator overloading

Question

I'm working on "Roman to int" algorithm, my code is below. I get an error:

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

Can somebody help me fix the code?

// 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;
}

If it's not Javascript/HTML/CSS, don't wrap it in a stack snippets. — user202729, Mar 06 '18 at 12:04
@Ramzan Chasygov Just write return one == ch; and the qualifier const to the operator == — Vlad from Moscow, Mar 06 '18 at 12:05
`one` is const, so you can call only const member, add const specifier to operator== definition. — rafix07, Mar 06 '18 at 12:07
[Potential dupe](https://stackoverflow.com/q/1188566/5267751) for half of the issue. The other is pointed out in @rafix07 's comment above. — user202729, Mar 06 '18 at 12:08
which is also solved in the "Comparison operator" section in [this answer](https://stackoverflow.com/a/4421719/5267751). — user202729, Mar 06 '18 at 12:09

score 2 · Accepted Answer · answered Mar 06 '18 at 12:10

2

At least declare the operator like

  bool operator ==(char ch) const {
    return ch == upperCase || ch == lowerCase;
  };

and use

return one == ch;

answered Mar 06 '18 at 12:10

Vlad from Moscow

301,070
26
186
335

Now I get new error, In function In function Solution::romanToInt()': main.cpp:undefined reference to Solution::one' collect2: error: ld returned 1 exit status – Rami Chasygov Mar 06 '18 at 12:14
That's a different matter, ask another question (research first please) – user202729 Mar 06 '18 at 12:16
@RamzanChasygov One problem is one question. Close this question and ask a new question relative to the new error. – Vlad from Moscow Mar 06 '18 at 12:16
@VladfromMoscow, ok I found my error, but what a deal with `const` in `bool operator ==(char ch) const { //... }` – Rami Chasygov Mar 06 '18 at 12:25
@VladfromMoscow, it makes return value `const`? – Rami Chasygov Mar 06 '18 at 12:26
1

@RamzanChasygov Because the object one is declared as a constant object static constexpr symbol one {'I', 'i'};. So the operator deals with a constant object and may apply only a constant function. – Vlad from Moscow Mar 06 '18 at 12:27

No operator '==' matches in operator overloading

1 Answers1