QtConcurrent::map() with member function = can not compile

Question

My project is to create a small program which demonstrates the work of a search engine: indexing and returning result for arbitrary queries. I\'ve done the work with the ind

Answer 1

To be able to call a pointer-to-member, you need, in addition to that functions formal arguments, an instance of that class (the this pointer that you get inside member functions).

There are two ways to handle this: create a simple functor to wrap the call, or use a lambda.

The functor would look like this:

struct AddWrapper {
  MainWindow *instance;
  AddWrapper(MainWindow *w): instance(w) {}
  void operator()(QString const& data) {
    instance->add(data);
  }
};

And you'd use it like:

AddWrapper wrap(this);
QtConcurrent::map(list, wrap);

(Careful with the lifetime of that wrapper though. You could make that more generic - you could also store a pointer-to-member in the wrapper for instance, and/or make it a template if you want to reuse that structure for other types.)

If you have a C++11 compiler with lambdas, you can avoid all that boilerpalte:

QtConcurrent::map(list, [this] (QString const& data) { add(data); });

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Note: I'm not sure how QtConcurrent::MemberFunctionWrapper1 got involved in your example, I'm not seeing it here. So there might be a generic wrapper already in Qt for this situation, but I'm not aware of it.