Problems with nested object in functional object of the tbb::flow::graph

Question

I have a functional object that I'm using as body for multifunction_node:

class module
{
private:
    bool valid;

    QString description;

    bool hasDetectionBranch;
    tDataDescription bufData;

    void* dllObject; //<-- This is a pointer to an object constructed with help of the external dll

    qint64 TimeOut;

public:

    module(const QString& _ExtLibName);
    virtual ~module();

    void operator() (pTransmitData _transmitData, multi_node::output_ports_type &op);
};

'dllObject' is created at construction time of the object 'module':

module::module(const QString& _ExtLibName) :
    valid(true), hasDetectionBranch(false)
{
    GetObjectDescription = (tGetObjectDescription)QLibrary::resolve(_ExtLibName, "GetObjectDescription");
    CreateObject = (tCreateObject)QLibrary::resolve(_ExtLibName, "CreateObject");
    DestroyObject = (tDestroyObject)QLibrary::resolve(_ExtLibName, "DestroyObject");

    if (!CreateObject || !DestroyObject || !GetObjectDescription)
        valid = false;
    else
    {
        description = QString(GetObjectDescription());
        dllObject = CreateObject();
    }
}

And this is when 'dllObject' is destroyed:

module::~module()
{
    if (valid)
    {
        DestroyObject(dllObject);
    }
}

I've built a little graph:

void MainWindow::goBabyClicked(void)
{
    module mod(QString("my.dll")); //<-- Here is OK and mod.dllObject is correct
    if (!mod.isValid())
    {
        qDebug() << "mod is invalid!\n";
        return;
    }

    first fir(input);
    folder fol(QString("C:/out"), 10000);

    graph g;

    source_node<pTransmitData> src(g, fir, false);

    multi_node mnode(g, tbb::flow::serial, mod); //<-- WTF? ~module() is executed!

    function_node<pTransmitData> f(g, tbb::flow::serial, fol);

    make_edge(src, mnode);
    make_edge(mnode, f);

    src.activate();

    g.wait_for_all();
}

So I have 2 questions: 1) Why ~module() is executed and how to prevent this? 2) How to keep pointer for nested object correctly?

UPDATE Added some dummy code to prevent destroying dllObject at first time like:

bool b = false;
module::~module()
{
    if (valid && b)
    {
        DestroyObject(dllObject);
    }
    if (!b)
        b = true;
    valid = false;
}

Now it works as expected but looks ugly :/

Answer 1

Max,

I assume you have a typedef of multi_node which is similar to the one in the reference manual example.

The constructor for the multifunction_node has the following signature:

multifunction_node( graph &g, size_t concurrency, Body body );

The body object is copied during the parameter passing and also during the construction of the node, so there are two copies of mod created during construction (actually three, as an initial copy of the body is also stored for re-initializing the body when calling reset() with rf_reset_bodies). The destructor calls you are seeing are probably those used to destroy the copies.

The body object should also have a copy-constructor defined or be able to accept the default-copy-constructor to make copies of the body. I think the QString has a copy-constructor defined, but I don't know about fields like tDataDescription. (I thought we had covered the basic requirements for Body objects in the Reference Manual, but I am still looking for the section.) In any case, the Body class must be CopyConstructible, as it is copied multiple times.

Regards, Chris