CODEWITHSUNDEEP
c++boostboost-signalsnoncopyableboost-signals2
User
User

Reputation: 65951

Error: "cannot access private member declared in class 'boost::signals2::scoped_connection'"?

class Whatever
{
public:
    virtual ~Whatever();

protected:
    Whatever();
    virtual void SomeMethod();
    void OnEventOccurred(int x);

    std::vector<boost::signals2::scoped_connection> boostSignalConnections_;
}

// .cpp

Whatever::SomeMethod()
{
    ...
    boostSignalConnections_.push_back(
        anObject->AddEventOccurredObserver(
            std::bind(&Whatever::OnEventOccurred,
                      this, std::placeholders::_1)));
    ...
}

(Note AddEventOccurredObserver just delegates to boost::signals2::connect() and returns the boost::signals2::connection)

I'm getting the following error. It's hard to interpret template errors but it seems like the error is being caused at the member declaration of std::vector<boost::signals2::scoped_connection> boostSignalConnections_;

...\vc\include\xmemory(202): error C2248:  
    'boost::signals2::scoped_connection::scoped_connection' :
    cannot access private member declared in class 'boost::signals2::scoped_connection'
...\boost_1_47\boost\signals2\connection.hpp(234) : see declaration of
'boost::signals2::scoped_connection::scoped_connection'
...\boost_1_47\boost\signals2\connection.hpp(210) :
    see declaration of 'boost::signals2::scoped_connection'
...\vc\include\xmemory(201) : while compiling class template member function
'void std::allocator<_Ty>::construct(boost::signals2::scoped_connection *,_Ty &&)'
with
[
    _Ty=boost::signals2::scoped_connection
]

I've searched and I think it might have to do with scoped_connection not being copyable? I'm not sure. The reason I'm using a scoped_connection is because of this SO question: Handle connection/disconnection of many signals/slots with boost::signals2

Update

FYI, when I change from scoped_connection to connection it works: std::vector<boost::signals2::connection> boostSignalConnections_;. My whole reason for using scoped_connection is that they automatically disconnect the connection on destruction whereas I believe a connection doesn't not. However, I can just iterate over the collection and manually disconnect each one.

Upvotes: 3

Views: 2366

Answers (1)

ildjarn
ildjarn

Reputation: 62975

boost::signals2::scoped_connection is non-copyable, and in C++03, std::vector<T> requires that T be both copy-constructible and copy-assignable (this requirement is relaxed in C++11, where a non-copyable type is fine as long as it's movable instead).

The naive solution would be to make boostSignalConnections_ a std::vector<scoped_connection*> instead, but this introduces object lifetime and exception-safety issues that aren't worth the effort. Instead, depending on how recent your compiler is, I'd recommend the following:

  • C++03: Use boost::ptr_vector<scoped_connection>, which will store pointers for you (allowing non-copyable types) but give you value-semantics, and eliminate the aforementioned object lifetime and exception-safety issues.
  • C++11: The above is still a valid option, but the more idiomatic approach would be to use std::vector<std::unique_ptr<scoped_connection>>.

That being said, why used scoped_connection if you don't want your object to be lexically-scoped?

Upvotes: 9

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