Insert function pointer into QMap (Qt)

Question

I am creating sort of a router for REST API in Qt and I am facing problem with inserting the function pointer into the QMap.

I have class IModule from which the other modules are derivated. Important lines of IModule.h are

typedef QByteArray* (*IBusAction)(IBus * , ProxyRequest *);

class IModule : public QObject
{
   Q_OBJECT

  protected:
    QMap<QString , IBusAction > *m_actions;

Then I have UserModule which is derived from IModule and in .cpp file I have these lines:

QByteArray* UserModule::create(IBus *bus, ProxyRequest *req)
{
}

QByteArray* UserModule::show( IBus *bus, ProxyRequest *req)
{
}


UserModule::UserModule(QObject *parent) :
IModule(parent)
{

    // register functions
    m_actions->insert("show", &UserModule::show);
    m_actions->insert("create", UserModule::create);

}

So I tried two options how to put the function in QMap with referencing sign also without it, but both are not working. I am getting error: no matching function for call to 'QMap<QString, QByteArray* (*)(IBus*, ProxyRequest*)>::insert(const char [5], QByteArray* (UserModule::*)(IBus*, ProxyRequest*))'

I have spent several hours with this problem, tried many different things how to solve it but there was no success.

So I will be very glad for any piece of advice.

Answer 1

Your IBusAction is a pointer-to-function type. This is not compatible with pointer-to-member function.

When you call a member function (like your UserModule::create function), it needs an extra ("invisible") parameter: the instance the function gets called on (the this pointer).

You have essentially three options:

• Change IBusAction to be a pointer-to-member-function-of-UserModule. You're restricted to that class's functions with this though.

  typedef QByteArray* (UserModule::*IBusAction)(IBus * , ProxyRequest *);
  

• Make the functions static (this changes the semantics of your code)

• Use free-standing functions (top-level, not members of a class) instead of member functions.

Answer 2

Pointers to member functions in C++ is generally more hassle than it is worth. Sane alternative is to have class instead of method, and keep pointer to instance instead of method.

Qt specific alternative is to use invokable methods of QObject based class, which you can call by string name, using QObject::invokeMethod, side-stepping many type problems.