CODEWITHSUNDEEP
c++multithreading
Paul
Paul

Reputation: 13

C++ make new thread function with default parameter

Heyo everyone, I'm currently playing around with threading in C++ and stumbled upon one quirk i couldn't quite find an answer to:

in my programm is a worker class which handles creating, and talking about a thread which does X.

worker::start(string filename){
   thread t(&worker::readfile,this,filename,10);
   t.detach();
}
worker::readfile(string filename, int LinesToRead = -1){
   stringstream ss = #open file and save as stringstream
   if(LinesToRead == -1){
      #read all lines from file
   else{
     #read "LinestoRead" lines from file
   }
   #does more with the stringstream to read it line by line, but not important here for the question.
}

this works fine, so does

thread t(&worker::readfile,this,filename,-1);

But why can i not call readfile as a thread with:

thread t(&worker::readfile,this, filename);

So if i leave the last variable "LinesToRead" blank it should default to -1, but i just get error messages when compiling. It's not really an issue, but I'd like to know... euhm why this is? Just calling worker::readfile(filename) works, since "LinesToRead" defaults to -1, but calling it as a thread all variables seem to be required.

Greetings

Upvotes: 1

Views: 194

Answers (2)

MSalters
MSalters

Reputation: 179819

In C++, there's a difference between default parameters and overloads, even though they can achieve similar effects. For instance, you could have written this instead:

class worker {
 void readfile(string filename, int LinesToRead);
 void readfile(string filename) { readfile(filename, -1); }
 // ...

This overload achieves the same effect: you can call it with one or two arguments, and if you call it with one it behaves as if the second argument was -1. With the overload, &worker::readfile would be ambiguous because there are two functions named readfile. In your default argument case, there's only one function readfile. Therefore, &worker::readfile is not ambiguous, but its type is void (worker::*)(string, int). The default argument is not part of the type.

Upvotes: 1

jhasse
jhasse

Reputation: 2593

The compiler will put in default arguments for you whenever the function is called. But in

thread t(&worker::readfile, this, filename);

the function call is inside the standard library. The thread constructor gets a function pointer which can only be used with all arguments.

You could get around this using a lambda:

thread t([this, filename]() { readfile(filename); });

Upvotes: 1

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