C++ operator= overloading

Question

I have struct Token, to which I'm trying assign operator=. I wanna be able to assign to char. I tried char operator=(const Token& token){ return token.kind; }, which throws error, says is not unary operator, tried char operator=(const char& ch, const Token& token){ return token.kind; } didn't help either. Yes I can do just char ch { token.kind };, but I wanna do it through operator in case if add some logic. Can you help me?

REPL EXAMPLE

struct Token {
  char kind;
  int value;

  Token(char kind, int value): kind(kind), value(value){}:
}

Please present a complete example that readers that can try out. — Cheers and hth. - Alf, Aug 01 '18 at 10:37
You want `Token::operator char() const;` in in your token class. Beware this can give unexpected results. Prefer `Token::as_char() const;` instead. — Richard Critten, Aug 01 '18 at 10:38
For the record, that explicit conversion is (IMO) much better. Adding implicit conversions to primitive types is the route to madness. Your logic that you want to give `Token` the power to decide how the conversion should be performed is reasonable, so a conversion operator makes some sense... but at least make it `explicit`! — Lightness Races in Orbit, Aug 01 '18 at 11:04

songyuanyao · Accepted Answer · 2018-08-01T11:08:08.387

7

You can't overload operator= for built-in types like char, to allow assign Token to char. operator= must be overloaded as member function.

As the workaround, you can add a conversion function, which allows implicit conversion from Token to char. Note that implicit conversion might cause potential issues, e.g. it allows to pass a Token to a function which expects a char.

struct Token {
  char kind;
  int value;

  Token(char kind, int value): kind(kind), value(value){}
  operator char() const { return kind; }
};

BTW: You can make the conversion function explicit, to avoid the issues of implicit conversion. And then in the assignment from Token to char you have to apply explicit conversion, like c = static_cast<char>(token);.

edited Aug 01 '18 at 11:08

answered Aug 01 '18 at 10:38

songyuanyao

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but why I can't use operator=, if c++ allows you, or is it only can return `Token`? – Rami Chasygov Aug 01 '18 at 10:46
@RamzanChasygov read the relevant part of this: https://stackoverflow.com/questions/4421706/what-are-the-basic-rules-and-idioms-for-operator-overloading – Richard Critten Aug 01 '18 at 10:47
@RamzanChasygov You can't overload `operator=` to accomplish this, I added some explanations at the beginning. And you can get more from the link I posted. – songyuanyao Aug 01 '18 at 10:49
@songyuanyao Sorry, I did't see it – Rami Chasygov Aug 01 '18 at 10:50
@RamzanChasygov That's fine, I added it later. – songyuanyao Aug 01 '18 at 10:52
1

For a bonus point recommend `explicit` (if supported) to fix the implicit conversion "problem". – Lightness Races in Orbit Aug 01 '18 at 11:05
@LightnessRacesinOrbit Thanks for the recommendation. – songyuanyao Aug 01 '18 at 11:09
but what if instead of struct I have enum? Is it possible to add conversation function? – Rami Chasygov Aug 01 '18 at 11:33
@RamzanChasygov You can't add conversion function to enum, afaik. Unscoped enum could implictly convert to integral types, while scoped enum only supports explicit conversion. – songyuanyao Aug 01 '18 at 11:42

C++ operator= overloading

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