指定mapreduce函数

Specifying reduce functor for mappedReduce

本文关键字：函数 mapreduce 指定      更新时间：2023-10-16

我正在使用QtConcurrent并行运行某个任务。然而，我遇到了一个问题，当我真的改变使用mapappereduced而不是map.

下面是我使用的代码: 
struct ProcMapWrapper {
  ConcurrentProcessTask *instance;
  ProcMapWrapper(ConcurrentProcessTask *w): instance(w) {}
  QString & operator()(const QString& data) {
    return instance->map(data);
  }
  ProcMapWrapper(const ProcMapWrapper & src)
  {
      instance = src.instance;
  }
  typedef QString result_type;
};
struct ProcReduceWrapper {
  ConcurrentProcessTask *instance;
  ProcReduceWrapper(ConcurrentProcessTask *w): instance(w) {}
  void operator()(int &number, const QString &fname) {
    return instance->reduce(number,fname);
  }
  ProcReduceWrapper(const ProcReduceWrapper & src)
  {
      instance = src.instance;
  }
  typedef int result_type;
};
ProcMapWrapper mw(this);
ProcReduceWrapper rw(this);
futureWatcher.setFuture(QtConcurrent::mappedReduced(_files.begin(),_files.end(),mw,rw));

这个想法是使用processOneItem函数从输入字符串生成stage1Data结构，然后在reduce函数中清除并将结果保存在适当的容器中。

我做了一些研究，使我找到了这个源文件，它引用了这些代码片段。在这里，map函子的模板如下:

struct Scaled 
{ 
     Scaled(int size) 
     : m_size(size) { } 
     typedef QImage result_type; 
     QImage operator()(const QImage &image) 
     { 
         return image.scaled(m_size, m_size); 
     } 
     int m_size; 
}; 
 QList<QImage> images = ...; 
 QFuture<QImage> thumbnails = QtConcurrent::mapped(images, Scaled(100));

这确实适用于QtConcurrent的map和mapped函数，但是我试图用相同的模式制作一个reduce函子，再次导致这些错误:

..qt_photoaudtask.cpp(230):错误C2893:未能专门化函数模板"QFuture: ResultType>QtConcurrent:: mappedReduced(迭代器,迭代器,MapFunctor ReduceFunctor, QtConcurrent:: ReduceOptions)"使用以下模板参数:"QList:: iterator"与(T = QString］"ProcMapWrapper"qt_photoaudtask.cpp(230):错误C2783: 'QFuture .cpp 'QtConcurrent:: mappedReduced(迭代器,迭代器,MapFunctor ReduceFunctor, QtConcurrent:: ReduceOptions)"无法推断出"ResultType"的模板参数c:qt4.8.4includeqtcore././src/corelib/concurrent/qtconcurrentmap.h(152):见声明'QtConcurrent:: mappereduced 'qt_photoaudtask.cpp(230):错误C2893:无法专门化函数模板"QFuture: ResultType>QtConcurrent:: mappedReduced (const序列,, MapFunctor ReduceFunctor, QtConcurrent:: ReduceOptions) '使用以下模板参数:"QList:: iterator"与(T = QString］"QList:: iterator"与(T = QString］'ProcMapWrapper' ..qt_photoaudtask.cpp(230):错误C2783: 'QFuture QtConcurrent:: mappereduced (const Sequence&，MapFunctor,ReduceFunctor,QtConcurrent::ReduceOptions)':不能推断"ResultType"的模板参数c:qt4.8.4includeqtcore././src/corelib/concurrent/qtconcurrentmap.h(125):见声明'QtConcurrent::mappedReduced'

reduce函子是否真的被定义和/或识别?QtPrivate::ReduceResultType::ResultType如何工作?它需要什么?

编辑:假设函数与boost::function兼容，我尝试在reduce functor中指定first_argument_type, second_argument_type, arg1_type和arg2_type。

我是这么做的:

struct concurrentProcessDBMapper
{
    typedef stage1Data result_type;
    stage1Data operator()(QString fname)
    {
        return concurrentProcessDBMap(fname);
    }
};
struct concurrentProcessDBReducer
{
    DatabaseConcurrentProcessTask *instance;
    concurrentProcessDBReducer(DatabaseConcurrentProcessTask *i)
    {
        instance = i;
    }
    void operator()(int & number, const stage1Data in)
    {
        //concurrentProcessDBReduce
        instance->reduce(number,in);
    }
};
...
    QFutureWatcher<int> futureWatcher;
    QList<QString> _filesInWork;
...
futureWatcher.setFuture(
     QtConcurrent::mappedReduced
         <int, QList<QString>, concurrentProcessDBMapper,concurrentProcessDBReducer>
                            (_filesInWork,concurrentProcessDBMapper(),concurrentProcessDBReducer(this)));
...

总结一下，对于像-(map)->-(reduce)->这样的类型转换，map函函数的result_type定义为stage1Data，而reduce函函数根本没有定义任何result_type。Future watcher将其类型定义为reduce结果(即它的第一个out参数的类型)——在我的例子中是int——而mapappedreduced的模板形参是显式定义的。

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