How to implement generic callbacks in C++

boost, boost-bind, c++

Solution

The callback should be stored as a `boost::function<void, std::string>`. Then you can use `boost::bind` to "convert" any other function signature to such an object, by binding the other parameters.

Example

I've not tried to compile this, but it should show the general idea anyways

void DoLongOperation(boost::function<void, const std::string&> callback)
{
  std::string result = DoSomeLengthyStuff();
  callback(result);
}


void CompleteRoutine1(const std::string&);
void CompleteRoutine2(int param, const std::string&);

// Calling examples
DoLongOperation(&CompleteRoutine1); // Matches directly
DoLongOperation(boost::bind(&CompleteRoutine2, 7, _1)); // int parameter is bound to constant.

// This one is thanks to David Rodríguez comment below, but reformatted here:
struct S 
{ 
  void f( std::string const & );
};

int main() 
{ 
  S s;
  DoLongOperation( boost::bind( &S::f, &s, _1 ) ); 
}

Problem

Forgive my ignorance in asking this basic question but I've become so used to using Python where this sort of thing is trivial that I've completely forgotten how I would attempt this in C++. I want to be able to pass a callback to a function that performs a slow process in the background, and have it called later when the process is complete. This callback could be a free function, a static function, or a member function. I'd also like to be able to inject some arbitrary arguments in there for context. (ie. Implementing a very poor man's coroutine, in a way.) On top of that, this function will always take a std::string, which is the output of the process. I don't mind if the position of this argument in the final callback parameter list is fixed. I get the feeling that the answer will involve boost::bind and boost::function but I can't work out the precise invocations that would be necessary in order to create arbitrary callables (while currying them to just take a single string), store them in the background process, and invoke the callable correctly with the string parameter.

Original source