具有成员函数=的QtConcurrent :: map()无法编译 [英] QtConcurrent::map() with member function = can not compile

问题描述

我的项目是创建一个小程序，该程序演示搜索引擎的工作:为任意查询建立索引并返回结果.我已经完成了索引器部分的工作，现在我想通过一次索引多个文件来改进它. MainWindow类在这里:

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 indexer part and now I want to improve it with indexing multiple files at once. The MainWindow class is here:

class MainWindow : public QMainWindow
{
    Q_OBJECT
    .....
private:
    Indexer * indexer;
    QStringList fileList;
    ....
    void index(QStringList list);
    void add(const QString &filename);
}

这是add的实现(add需要访问fileList以避免再次索引同一文件，因此不能是静态方法):

This is the implementation of add (add need to access fileList to avoid index the same files again, thus it can not be static method):

void MainWindow::add(const QString &filename)
{
    if (!fileList.contains(filename))
    {
        indexer->addDocument(filename.toStdString());
        fileList.append(filename);
        qDebug() << "Indexed" << filename;
        emit updatedList(fileList);
    }
}

index方法的实现是接收文件列表，并在每个文件名上调用add:

The implement of index method is to receive a file lists and call add upon each file name:

void MainWindow::index(QStringList list)
{
    ....
    QtConcurrent::map(list, &MainWindow::add);
    ....
}

我在编译这些代码时收到的错误是:

The error I receive when compiling these code is:

usr/include/qt4/QtCore/qtconcurrentmapkernel.h: In member function 'bool QtConcurrent::MapKernel<Iterator, MapFunctor>::runIteration(Iterator, int, void*) [with Iterator = QList<QString>::iterator, MapFunctor = QtConcurrent::MemberFunctionWrapper1<void, MainWindow, const QString&>]':
../search-engine/mainwindow.cpp:361:1:   instantiated from here
/usr/include/qt4/QtCore/qtconcurrentmapkernel.h:73:9: error: no match for call to '(QtConcurrent::MemberFunctionWrapper1<void, MainWindow, const QString&>) (QString&)'
/usr/include/qt4/QtCore/qtconcurrentfunctionwrappers.h:128:7: note: candidate is:
/usr/include/qt4/QtCore/qtconcurrentfunctionwrappers.h:138:14: note: T QtConcurrent::MemberFunctionWrapper1<T, C, U>::operator()(C&, U) [with T = void, C = MainWindow, U = const QString&]
/usr/include/qt4/QtCore/qtconcurrentfunctionwrappers.h:138:14: note:   candidate expects 2 arguments, 1 provided

我不太了解QtConcurrent的工作原理，并且文档中没有提供很多有关它的细节.我真的希望这里有人可以提供帮助.预先感谢.

I'm not really familiar with how QtConcurrent works, and the documentation doesn't provide much details about it. I really hope that someone here can help. Thanks in advance.

推荐答案

要能够调用指向成员的指针，除了需要使用函数的形式参数之外，还需要该类的实例(this成员函数内部的指针).

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).

有两种处理方法:创建一个简单的函子来包装调用，或使用lambda.

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

函子看起来像这样:

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

您将像这样使用它:

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

(但是请注意该包装器的生命周期.您可以使它更通用-例如，也可以在包装器中存储指向成员的指针，并且/或者如果要重用该结构，可以将其设为模板对于其他类型.)

(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.)

如果您具有带有lambdas的C ++ 11编译器，则可以避免所有这些样板:

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

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

---

注意:我不确定QtConcurrent::MemberFunctionWrapper1如何参与您的示例，在这里看不到.因此，在这种情况下，Qt中可能已经有一个通用包装器，但是我不知道.

---

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.

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