Using lambdas with QtConcurrent::blockingMappedReduced does not compile

Hello, I've been trying to get QtConcurrent::blockingMappedReduced to run using a couple of lambda's and keep running into a compile issue which I can't get to the bottom of. I've cut it right back and am currently using Qt's own example code converted into lambda's and getting the same error. So here it is:

Code:

---

typedef QMap<QString, int> WordCount;
 
void ClassName::ProcessStuff() {
   auto wordMapFn = [&](const QString &file) -> WordCount {
      QFile f(file);
      f.open(QIODevice::ReadOnly);
      QTextStream textStream(&f);
      WordCount wordCount;
 
      while (textStream.atEnd() == false)
         foreach (QString word, textStream.readLine().split(" "))
         wordCount[word] += 1;
 
      return wordCount;
   };
 
   auto wordReduceFn = [&](WordCount &result, const WordCount &w) {
      QMapIterator<QString, int> i(w);
      while (i.hasNext()) {
         i.next();
         result[i.key()] += i.value();
      }
   };
   QStringList files;
   WordCount total = QtConcurrent::blockingMappedReduced(files.begin(), files.end(), wordMapFn, wordReduceFn);
}

---

Compiling this in VS2012 with Qt5.0.2 produces the following error:

Code:

---

1>Implementation\ImportExport\OpcExporter.cpp(250): error C2893: Failed to specialize function template 'QtPrivate::ReduceResultType<ReduceFunctor>::ResultType QtConcurrent::blockingMappedReduced(Iterator,Iterator,MapFunctor,ReduceFunctor,QtConcurrent::ReduceOptions)'
1>          With the following template arguments:
1>          'QList<T>::iterator'
1>          with
1>          [
1>              T=QString
1>          ]
1>          'ImportExport::OpcExporter::CreateProjectChildAndAttachedParts::<lambda_34d77ea4bb1263c3043c2eae1f2efb26>'
1>          'ImportExport::OpcExporter::CreateProjectChildAndAttachedParts::<lambda_0b977c9c36b310423ccf2f5f1c540fb3>'
1>Implementation\ImportExport\OpcExporter.cpp(250): error C2783: 'ResultType QtConcurrent::blockingMappedReduced(Iterator,Iterator,MapFunctor,ReduceFunctor,QtConcurrent::ReduceOptions)' : could not deduce template argument for 'ResultType'
1>          c:\qt\qt5.0.2\5.0.2\msvc2012_64\include\qtconcurrent\qtconcurrentmap.h(237) : see declaration of 'QtConcurrent::blockingMappedReduced'
1>Implementation\ImportExport\OpcExporter.cpp(250): error C2893: Failed to specialize function template 'QtPrivate::ReduceResultType<ReduceFunctor>::ResultType QtConcurrent::blockingMappedReduced(const Sequence &,MapFunctor,ReduceFunctor,QtConcurrent::ReduceOptions)'
1>          With the following template arguments:
1>          'QList<T>::iterator'
1>          with
1>          [
1>              T=QString
1>          ]
1>          'ImportExport::OpcExporter::CreateProjectChildAndAttachedParts::<lambda_34d77ea4bb1263c3043c2eae1f2efb26>'
1>          'QList<T>::iterator'
1>          with
1>          [
1>              T=QString
1>          ]
1>Implementation\ImportExport\OpcExporter.cpp(250): error C2783: 'ResultType QtConcurrent::blockingMappedReduced(const Sequence &,MapFunctor,ReduceFunctor,QtConcurrent::ReduceOptions)' : could not deduce template argument for 'ResultType'
1>          c:\qt\qt5.0.2\5.0.2\msvc2012_64\include\qtconcurrent\qtconcurrentmap.h(208) : see declaration of 'QtConcurrent::blockingMappedReduced'
1>
1>Build FAILED.

---

Any help much appreciated.

Try calling with the template type explicitly specified, I guess something like

Code:

---

WordCount total = QtConcurrent::blockingMappedReduced<WordCount>(files.begin(), files.end(), wordMapFn, wordReduceFn);

---

Cheers,
_

Ah of course. Yes that worked. Thanks.

I spoke too soon:

Code:

---

void MyClass::MyFunc() {
   typedef QMap<QString, int> WordCount;
 
 
   auto wordMapFn = [&](const QString &file) -> WordCount {
      QFile f(file);
      f.open(QIODevice::ReadOnly);
      QTextStream textStream(&f);
      WordCount wordCount;
 
      while (textStream.atEnd() == false)
         foreach (QString word, textStream.readLine().split(" "))
         wordCount[word] += 1;
 
      return wordCount;
   };
 
   auto wordReduceFn = [&](WordCount &result, const WordCount &w) {
      QMapIterator<QString, int> i(w);
      while (i.hasNext()) {
         i.next();
         result[i.key()] += i.value();
      }
   };
   QStringList files;
   WordCount total = QtConcurrent::blockingMappedReduced<WordCount>(files.begin(), files.end(), wordMapFn, wordReduceFn);
}

---

gives me this error:

Code:

---

1>c:\qt\qt5.0.2\5.0.2\msvc2012_64\include\qtconcurrent\qtconcurrentmapkernel.h(116): error C2039: 'result_type' : is not a member of 'ImportExport::MyClass::MyFunc::<lambda_fa3d57d9b3ee2c79a98275b7637f0691>' (Implementation\ImportExport\MyClass.cpp)
...

---

The problem is that you can't use the auto keyword for the lambda's or it gets confused. So you have to fully specify the lambdas type:

Code:

---

std::function<WordCount (const QString&)> wordMapFn = [&](const QString &file) -> WordCount {
      ...
   };

---

Hope this helps others.

Could also be a temporary compiler limitation. The C++11 stuff is relatively new.

Cheers,
_

For future folks referring to this thread, I found that the std::function<> requirement was definitely and only true if the lambda had a return type, such as for the Mapped functor. Because the return type of the Reduced functor was void, std::function wasn't required.

I was compiling with MSVC2010.