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:

Add later to your DESCRIPTION file, under both LinkingTo and Imports
Make sure that your NAMESPACE file has an import(later) entry. If your package uses roxygen2, you can do this by adding the following lines to any file under R/:

#' @import later
NULL

Add #include <later_api.h> to the top of each C++ file that uses the below APIs.

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.

Using later from C++

Joe Cheng

2024-11-08