Using later from C++

Joe Cheng

2021-04-20

Using later from C++

You can call later::later from C++ code in your own packages, to cause your own C-style functions to be called back. This is safe to call from either the main R thread or a different thread; in both cases, your callback will be invoked from the main R thread.

To use the C++ interface, you’ll need to:

#' @import later
NULL

Executing a C function later

The later::later function is accessible from later_api.h and its prototype looks like this:

void later(void (*func)(void*), void* data, double secs)

The first argument is a pointer to a function that takes one void* argument and returns void. The second argument is a void* that will be passed to the function when it’s called back. And the third argument is the number of seconds to wait (at a minimum) before invoking. In all cases, the function will be invoked on the R thread, when no user R code is executing.

Background tasks

This package also offers a higher-level C++ helper class called later::BackgroundTask, to make it easier to execute tasks on a background thread. It takes care of launching the background thread for you, and returning control back to the R thread at a later point; you’re responsible for providing the actual code that executes on the background thread, as well as code that executes on the R thread before and after the background task completes.

Its public/protected interface looks like this:

class BackgroundTask {

public:
  BackgroundTask();
  virtual ~BackgroundTask();

  // Start executing the task  
  void begin();

protected:
  // The task to be executed on the background thread.
  // Neither the R runtime nor any R data structures may be
  // touched from the background thread; any values that need
  // to be passed into or out of the Execute method must be
  // included as fields on the Task subclass object.
  virtual void execute() = 0;
  
  // A short task that runs on the main R thread after the
  // background task has completed. It's safe to access the
  // R runtime and R data structures from here.
  virtual void complete() = 0;
}

Create your own subclass, implementing a custom constructor plus the execute and complete methods.

It’s critical that the code in your execute method not mutate any R data structures, call any R code, or cause any R allocations, as it will execute in a background thread where such operations are unsafe. You can, however, perform such operations in the constructor (assuming you perform construction only from the main R thread) and complete method. Pass values between the constructor and methods using fields.

#include <Rcpp.h>
#include <later_api.h>

class MyTask : public later::BackgroundTask {
public:
  MyTask(Rcpp::NumericVector vec) :
    inputVals(Rcpp::as<std::vector<double> >(vec)) {
  }

protected:
  void execute() {
    double sum = 0;
    for (std::vector<double>::const_iterator it = inputVals.begin();
      it != inputVals.end();
      it++) {
      
      sum += *it;
    }
    result = sum / inputVals.size();
  }
  
  void complete() {
    Rprintf("Result is %f\n", result);
  }

private:
  std::vector<double> inputVals;
  double result;
};

To run the task, new up your subclass and call begin(), e.g. (new MyTask(vec))->begin(). There’s no need to keep track of the pointer; the task object will delete itself when the task is complete.

// [[Rcpp::export]]
void asyncMean(Rcpp::NumericVector data) {
  (new MyTask(data))->begin();
}

It’s not very useful to execute tasks on background threads if you can’t get access to the results back in R. We’ll soon be introducing a complementary R package that provides a suitable “promise” or “future” abstraction.