C++ Unit Tests for R Packages

Are you familiar with testthat? It’s another package from the Hadleyverse that makes it easy (and fun!) to write unit tests for your R code. The tests you write look something like this:

context("Package Feature")

test_that("Feature works as expected", {
  number <- package::two_plus_two()
  expect_true(number == 4)
})

This is a great interface for testing R code… but code in a package isn’t always R code. Wouldn’t it be nice if we could do something like this with our C / C++ code, as well?

There are in fact a number of libraries for unit testing of C / C++ code, but there exists one that has a surprisingly similar interface to testthat – Catch. Catch lets you write unit tests of the form:

#include "catch.hpp"

int twoPlusTwo() {
  return 2 + 2;
}

TEST_CASE("Two plus two is four", "[arith]") {
  REQUIRE(twoPlusTwo() == 4);
}

This looks surprisingly testthat-like, but it’s not quite there. There’s also the baggage of figuring out how to compile and run the test executable, which is just not fun.

Fortunately, the development version of testthat now bundles Catch and makes it super easy to create and run Catch unit tests as a part of your regular R package development workflow. Give it a shot in your own package:

devtools::install_github("hadley/testthat")
testthat::use_catch()

This will add the necessary test infrastructure to your package. And now, voila, when you test your package (say, by pressing Ctrl + Shift + T in RStudio), any Catch unit tests found in the the C++ files contained in your src/ folder will automatically be run. How slick is that?

The format of your C++ unit tests is (using some handy #defines) is of the form:

#include <testthat.h>

int twoPlusTwo() {
  return 2 + 2;
}

context("Arithmetic") {
  test_that("Two plus two is four") {
    expect_true(twoPlusTwo() == 4);
  }
}

That’s virtually identical to the R code formulation, barring some changes in where the braces show up. You can see how testthat itself tests some example code here.

With this, I hope that unit testing of compiled code in an R package will become just as easy (and fun) as testing of the R code itself. Test and be happy!