比较boost :: optional <T&> const T＆

Question

I'm trying to compare a constant reference and a non-constant optional object with the same type. I have a type, NonCopy , which is noncopyable

#include <iostream>
#include <boost/optional.hpp>

struct NonCopy {
    NonCopy() { }
    NonCopy(const NonCopy&) = delete;
    NonCopy& operator=(const NonCopy&) = delete;
};

int main()
{
    NonCopy nc;
    const NonCopy& object = nc;
    boost::optional<NonCopy&> object2 = nc;

    if (!object2 && object2.get() != object) {
        std::cout << "not equal?\n";
    }
}

Which yields

error: no match for ‘operator!=’ (operand types are ‘boost::optional_detail::types_when_is_ref<NonCopy&>::raw_type {aka NonCopy}’ and ‘const NonCopy’)

I've tried multiple variations on the theme, including

if (object2 && const_cast<const NonCopy&>(object2.get()) != object)

Which yields a very interesting error of

error: no match for ‘operator!=’ (operand types are ‘const NonCopy’ and ‘const NonCopy’)

and lists candidates for != on boost::optional<NonCopy> (such as bool boost::operator!=(const boost::optional<NonCopy>&, const boost::optional<NonCopy>&) ), rather than on NonCopy .

Answer 1

Since object2.get() returns a NonCopy& , your example effectively simplifies to:

NonCopy nc;
const NonCopy& object = nc;
NonCopy& object2 = nc;

object != object2; // error: no match for operator!=

That simply means that your type NonCopy has no operator!= . Implement that, and your code will compile. boost::optional is not relevant here.

Although note that you're checking:

if (!object2 && object2.get() != object)
    ^^^^^^^^    ^^^^^^^^^^^^^
    object2 is none, but get it anyway?

That is undefined behavior.