CODEWITHSUNDEEP
c++multithreadingc++11
Sungmin
Sungmin

Reputation: 2631

Passing arguments to thread function (templated)

This question might be related to Why does passing object reference arguments to thread function fails to compile?.

I encountered a similar problem, however, in my case the functor is a template. 

class A {
public:
  // Non template version works as expected!!.
  // void operator()(std::ostream& out){
  //    out << "hi\n";
  // }

  // template version doesn't. 
    template <class Ostream>
    void operator()(Ostream& out){
        out << "hi\n";
    }

};

int main() {
   A a;
   thread t(a, ref(cout));
   t.join();
}

GCC says:

error: no match for 'operator<<' in 'out << "hi\012"'

How can I solve this problem?

Upvotes: 4

Views: 286

Answers (1)

Karthik T
Karthik T

Reputation: 31952

You are passing a std::reference_wrapper. So the type of class Ostream will be std::reference_wrapper which explains the error.

template <class OstreamRef>
void operator()(OstreamRef& outRef){
    outRef.get()<< "hi\n";
}

This should fix it.

With a non template case, when it needs to convert to std::ostream&, the get() is implicitly called. However, with a template there is no need to convert to any other type, so the std::reference_wrapper is passed as-is and thus needs an explicit call to get(). Thanks @jogojapan

Upvotes: 3

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