Question

Given the code in Visual Studio 2010:

void Foo::Bar() const
{
    map_t::const_iterator iter(my_map_.find(key));

    if(my_map_.end() != iter)
        DoStuff(iter->second);
}

Do stuff will take the mapped_type by value. The mapped type is a copyable, movable, assignable type.

I get error messages that the key/value pair can’t be copied when trying to access second. Even if I write on their own lines:

iter->second;
(*iter).second;

to ensure it’s nothing to do with DoStuff…

I presume the iterator is making a copy of the key/value pair before returning operator-> or operator*.

How do I get a copy of the mapped type?

EDIT:

The map itself is of unsigned shorts to boost variants, roughly as such:

typedef struct{} empty_t;
typedef boost::variant<empty_t, double, long, some POD types> variant_t;
typedef std::map<unsigned short, variant_t> map_t;

And then as a private member of the class:

map_t my_map_;

And to be clear, the problem is not in passing to DoStuff. I can remove that line, and simply dereference the iterator and access second, and that will cause the compiler error.

if(my_map_.end() != iter)
        iter->second; //Doesn't do anything, but illustrates the error.

Answer 1

I figured it out.

WAYYYYY away from this code, and a couple abstractions deep, the iterators to the map were being passed to std::partition! You can’t partition a map, it seems.

I copied the contents of the map into a local vector and continued processing from there, and it cleared up all the error messages.

I have no idea why the compiler was getting so throughly confused. The problem code was somewhere completely unrelated. But it doesn’t matter.