boost::optional with const members

Question

Why doesn't this work?

struct O {
    O(int i, int j)
        : i(i)
        , j(j)
    {}

    int const i;
    int const j;
};

int main(int argc, char** argv)
{
    boost::optional<O> i;
    i.reset(O(4, 5));
    return 0;
}

It seems to be trying to use the assignment operator instead of trying to construct it in place. I had assumed it would call the copy constructor of O on uninitialized memory.

/..../include/boost/optional/optional.hpp:433:69: error: use of deleted function ‘O& O::operator=(const O&)’
.... error: ‘O& O::operator=(const O&)’ is implicitly deleted because the default definition would be ill-formed:
.... error: non-static const member ‘const int O::i’, can’t use default assignment operator
.... error: non-static const member ‘const int O::j’, can’t use default assignment operator

Bart van Ingen Schenau · Accepted Answer · 2013-01-17T08:58:21.570

2

Boost.Optional uses either assignment or copy-construction, depending on the state of i. As this state is run-time information, the choice between assignment and copy-construction also has to be made at run-time.
This means, however, that the compiler has to generate code for both options, even if one of them is never actually used. And that means that both options must be possible.

To get the code working, you could add an (always failing) assingment operator to class O:

O& O::operator=(const O&)
{
    throw "this is not possible"
    return *this;
}

As a side note, Optional<T>::reset is deprecated. You should just use assingment, as in

i = O(4,5);

The above described semantics are valid for both.

edited Jan 17 '13 at 08:58

answered Jan 17 '13 at 08:47

Bart van Ingen Schenau

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In which cases would this be used? When I try to assign to the optional? – nishantjr Jan 17 '13 at 08:49
@njr: The assignment operator would be used when you assign to an `Optional` that already holds a value. – Bart van Ingen Schenau Jan 17 '13 at 08:55
Ahhh... It fails if I reset twice too – nishantjr Jan 17 '13 at 09:01
Do you know why didn't they implement it as calling the destructor and and then the constructor again? – nishantjr Jan 17 '13 at 09:03
1

@njr: See http://www.boost.org/doc/libs/1_52_0/libs/optional/doc/html/boost_optional/development.html#boost_optional.development.the_interface – Bart van Ingen Schenau Jan 17 '13 at 09:09

boost::optional with const members

1 Answers1