how to divide boost::optional?

Question

I have such code:

boost::optional<double> result = _ind1.Value() / _ind2.Value();

Each arg is boost::optional<double> too:

boost::optional<double> Value() {
    return value;
}

Errors are:

Error 1 error C2676: binary '/' : 'boost::optional<T>' does not define this operator or a conversion to a type acceptable to the predefined operator 2 IntelliSense: no operator "/" matches these operands operand types are: boost::optional<double> / boost::optional<double>

I understand that it seems that division is just not defined. I expect result to be boost::none if any of two arguments is none - otherwise I want it to be normal double division. Should I just write this myself?

Zan Lynx · Accepted Answer · 2014-04-23T17:40:21.337

11

Of course such a simple operation as division of doubles is supported.

But you aren't trying to divide doubles. You're trying to divide boost::optional<double>s which is a whole different story.

If you want, you can define a division operator for this. It might look like (untested):

template<typename T>
boost::optional<T> operator/(const boost::optional<T>& a, const boost::optional<T>& b)
{
    if(a && b) return *a / *b;
    else return boost::optional<T>();
}

In C++11 (code courtesy of Yakk):

template<class T,class U> struct divide_result {
  typedef typename std::decay<decltype(std::declval<T>()/std::declval<U>())>::type;
};
template<class T, class U> using divide_result_t=typename divide_result<T,U>::type;
template<typename T,typename U>
boost::optional<divide_result_t<T,U>> operator/(const boost::optional<T>& a, const boost::optional<U>& b)
{
    if(a && b) return *a / *b;
    else return boost::none;
}

I used a template because now it's also good for int, float, etc.

edited Apr 23 '14 at 17:40

answered Apr 23 '14 at 16:46

Zan Lynx

53,022
10
79
131

Do you need to convert the return value to a `boost::optional`? or is there an implicit conversion? – Ferruccio Apr 23 '14 at 16:48
@Ferruccio: It has an implicit conversion. – Zan Lynx Apr 23 '14 at 16:54
what, no `T`/`U` support with automatic promotion? – Yakk - Adam Nevraumont Apr 23 '14 at 17:24
@Yakk: You can write the upgraded version. :-) – Zan Lynx Apr 23 '14 at 17:25
@zanlynx sure, make me write code on my phone. You can add SFINAE support if you want. – Yakk - Adam Nevraumont Apr 23 '14 at 17:32
Overall it looks quite complicated. Now i think probably it would be easier less error phrone and faster just use "double + bool" fields and do the rest myself? – Oleg Vazhnev Apr 23 '14 at 18:13
@javapowered: My version isn't that complicated. You'd only need the C++11 version by Yakk for dividing more complicated things, like a double by a complex or something like that. – Zan Lynx Apr 23 '14 at 18:19
@javapowered: or just take the if statement out of the above code and use it directly. – Zan Lynx Apr 23 '14 at 18:20
1

The C++11 version supports `optional/optional` where the types do not match (ie, one is `int` and another `double` – Yakk - Adam Nevraumont Apr 23 '14 at 19:30

how to divide boost::optional?

1 Answers1

Linked