Does boost::optional trigger a ref count on shared_ptr?

boost, c++, shared-ptr

Solution

Yes, it has to. `boost::optional` stores a copy, so for a `shared_ptr`, this means there is a copy of the `shared_ptr` and that means the reference count must be increased.

Note that as long as the `boost::optional` is empty, i.e., it doesn't contain a value of `shared_ptr`, there is no object whose reference count is fiddled with. In other words, an empty `boost::optional` does not contain an (empty or otherwise) `shared_ptr`.

The requested "semantics" can't really work because you keep one `shared_ptr` in the map and you return a `shared_ptr`.

However, you may return a `boost::optional<const V&>`:

boost::optional<const V&> findValue(const K& key) {
    auto it = map.find(key);
    if (it != map.end()) {
        return boost::optional<const V&>( it->second );
    }
    return boost::optional<const V&>();
}

but make sure that the reference remains valid while you keep/use it.

Problem

I'm trying to get a function to return an optional value from my map. So something like this: ``` boost::optional<V> findValue(const K& key) { boost::optional<V> ret; auto it = map.find(key); if (it != map.end()) { ret = it->second; } return ret; } ``` If `V` happens to be a `shared_ptr` type of some kind, does the assignment to `ret` trigger a reference count?

Original source