CODEWITHSUNDEEP
c++multithreadingthread-safetypthreads
robo
robo

Reputation: 23

pass std::map object to thread

Mycode is trying to pass a std::map as reference to thread but it seems something is bad and result in 

error: invalid conversion from ‘void* (*)(std::map<std::basic_string<char>,
       std::vector<std::basic_string<char> > >)’ to ‘void* (*)(void*)’ [-fpermissive]

I need to pass map to thread and insert the key and value of map in that thread and after successful. In main process i need to update or copy(thread map) in another object of same map i.e myMapcache 

int main()
{
std::map< std::pair<std::string , std::string> , std::vector<std::string> > myMap,myMapCache;

  pthread_t threads;

  //how to pass map object in thread as reference

  int rc = pthread_create(&threads, NULL, myfunction, std::ref(myMap)); 
  if (rc)
  {
     cout << "Error:unable to create thread," << rc << endl;
     exit(-1);
   }

   // if true then update to local myMapCache

   if(update)
    {
      std::copy(myMap.begin(), myMap.end(), std::inserter(MyMapCache, myMapCache.end()) );
    } 

}


void * myfunction (std::map< std::pair<std::string , std::string> , std::vector<std::string> >& myMap)
{

  // here i will insert data in a map
  myMap[std::make_pair(key1,key2)].push_back(value);

  // if update make the flag true
    Update=true;  



}

Upvotes: 0

Views: 1091

Answers (1)

Lightness Races in Orbit
Lightness Races in Orbit

Reputation: 385098

pthread_create is not a template, and it does not understand C++ types. It takes a void*, which is what C libraries do in order to fake templates (kind of).

You can pass a casted pointer instead of a C++ reference wrapper object:

int rc = pthread_create(&threads, NULL, myfunction, static_cast<void*>(&myMap)); 
// ...
void* myfunction(void* arg)
{
   using T = std::map<std::pair<std::string, std::string>, std::vector<std::string>>;
   T& myMap = *static_cast<T*>(arg);

…or, better yet, use boost::thread (C++98) or std::thread (C++11 and later) to get type safety and a longer lifespan. You're not writing a C program.

Upvotes: 5

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