Below are some Frequently Asked Questions about cpp11. If you have a question that you think would fit well here please open an issue.
1. What are the underlying types of cpp11 objects?
vector | element |
---|---|
cpp11::integers | int |
cpp11::doubles | double |
cpp11::logicals | cpp11::r_bool |
cpp11::strings | cpp11::r_string |
cpp11::raws | uint8_t |
cpp11::list | SEXP |
2. How do I add elements to a named list?
Use the push_back()
method with the named literal
syntax. The named literal syntax is defined in the
cpp11::literals
namespace.
#include <cpp11.hpp>
[[cpp11::register]]
::list foo_push() {
cpp11using namespace cpp11::literals;
::writable::list x;
cpp11.push_back({"foo"_nm = 1});
x
return x;
}
3. Does cpp11 support default arguments?
cpp11 does not support default arguments, while convenient they would
require more complexity to support than is currently worthwhile. If you
need default argument support you can use a wrapper function around your
cpp11 registered function. A common convention is to name the internal
function with a trailing _
.
#include <cpp11.hpp>
[[cpp11::register]]
double add_some_(double x, double amount) {
return x + amount;
}
add_some <- function(x, amount = 1) {
add_some_(x, amount)
}
add_some(1)
#> [1] 2
add_some(1, amount = 5)
#> [1] 6
5. How do I retrieve (named) elements from a named vector/list?
Use the []
accessor function.
x["foo"]
6. How can I tell whether a vector is named?
Use the named()
method for vector classes.
#include <cpp11.hpp>
[[cpp11::register]]
bool is_named(cpp11::strings x) {
return x.named();
}
is_named("foo")
#> [1] FALSE
is_named(c(x = "foo"))
#> [1] TRUE
7. How do I return a cpp11::writable::logicals
object
with only a FALSE
value?
You need to use list
initialization with {}
to create the object.
#include <cpp11.hpp>
[[cpp11::register]]
::writable::logicals my_false() {
cpp11return {FALSE};
}
[[cpp11::register]]
::writable::logicals my_true() {
cpp11return {TRUE};
}
[[cpp11::register]]
::writable::logicals my_both() {
cpp11return {TRUE, FALSE, TRUE};
}
my_false()
#> [1] FALSE
my_true()
#> [1] TRUE
my_both()
#> [1] TRUE FALSE TRUE
8. How do I create a new empty environment?
To do this you need to call the base::new.env()
function
from C++. This can be done by creating a cpp11::function
object and then calling it to generate the new environment.
#include <cpp11.hpp>
[[cpp11::register]]
::environment create_environment() {
cpp11::function new_env(cpp11::package("base")["new.env"]);
cpp11return new_env();
}
9. How do I assign and retrieve values in an environment? What happens if I try to get a value that doesn’t exist?
Use []
to retrieve or assign values from an environment
by name. If a value does not exist it will return
R_UnboundValue
.
#include <cpp11.hpp>
[[cpp11::register]]
bool foo_exists(cpp11::environment x) {
return x["foo"] != R_UnboundValue;
}
[[cpp11::register]]
void set_foo(cpp11::environment x, double value) {
["foo"] = value;
x}
x <- new.env()
foo_exists(x)
#> [1] FALSE
set_foo(x, 1)
foo_exists(x)
#> [1] TRUE
10. How can I create a cpp11:raws
from a
std::string
?
There is no built in way to do this. One method would be to
push_back()
each element of the string individually.
#include <cpp11.hpp>
[[cpp11::register]]
::raws push_raws() {
cpp11std::string x("hi");
::writable::raws out;
cpp11
for (auto c : x) {
.push_back(c);
out}
return out;
}
push_raws()
#> [1] 68 69
11. How can I create a std::string
from a
cpp11::writable::string
?
Because C++ does not allow for two implicit cast, explicitly cast to
cpp11::r_string
first.
#include <cpp11.hpp>
#include <string>
[[cpp11::register]]
std::string my_string() {
::writable::strings x({"foo", "bar"});
cpp11std::string elt = cpp11::r_string(x[0]);
return elt;
}
12. What are the types for C++ iterators?
The iterators are ::iterator
classes contained inside
the vector classes. For example the iterator for
cpp11::doubles
would be
cpp11::doubles::iterator
and the iterator for
cpp11::writable::doubles
would be
cpp11::writable::doubles::iterator
.
13. My code has using namespace std
, why do I still
have to include std::
in the signatures of
[[cpp11::register]]
functions?
The using namespace std
directive will not be included
in the generated code of the function signatures, so they still need to
be fully qualified. However you will not need to qualify the
type names within those functions.
The following won’t compile
#include <cpp11.hpp>
#include <string>
using namespace std;
[[cpp11::register]]
() {
string foobarreturn string("foo") + "-bar";
}
But this will compile and work as intended
#include <cpp11.hpp>
#include <string>
using namespace std;
[[cpp11::register]]
std::string foobar() {
return string("foo") + "-bar";
}
14. How do I modify a vector in place?
In place modification breaks the normal semantics of R code. In
general it should be avoided, which is why cpp11::writable
classes always copy their data when constructed.
However if you are positive in-place modification is necessary for your use case you can use the move constructor to do this.
#include <cpp11.hpp>
[[cpp11::register]]
void add_one(cpp11::sexp x_sexp) {
::writable::integers x(std::move(x_sexp.data()));
cpp11for (auto&& value : x) {
++value;
}
}
15. Should I call cpp11::unwind_protect()
manually?
cpp11::unwind_protect()
is cpp11’s way of safely calling
R’s C API. In short, it allows you to run a function that might throw an
R error, catch the longjmp()
of that error, promote it to
an exception that is thrown and caught by a try/catch that cpp11 sets up
for you at .Call()
time (which allows destructors to run),
and finally tells R to continue unwinding the stack now that the C++
objects have had a chance to destruct as needed.
Since cpp11::unwind_protect()
takes an arbitrary
function, you may be wondering if you should use it for your own custom
needs. In general, we advise against this because this is an extremely
advanced feature that is prone to subtle and hard to debug issues.
Destructors
The following setup for test_destructor_ok()
with a
manual call to unwind_protect()
would work:
#include <cpp11.hpp>
class A {
public:
~A();
};
::~A() {
A("hi from the destructor!");
Rprintf}
[[cpp11::register]]
void test_destructor_ok() {
{};
A a::unwind_protect([&] {
cpp11("oh no!");
Rf_error});
}
[[cpp11::register]]
void test_destructor_bad() {
::unwind_protect([&] {
cpp11{};
A a("oh no!");
Rf_error});
}
test_destructor_ok()
#> Error: oh no!
#> hi from the destructor!
But if you happen to move a
into the
unwind_protect()
, then it won’t be destructed, and you’ll
end up with a memory leak at best, and a much more sinister issue if
your destructor is important:
test_destructor_bad()
#> Error: oh no!
In general, the only code that can be called within
unwind_protect()
is “pure” C code or C++ code that only
uses POD (plain-old-data) types and no exceptions. If you mix complex
C++ objects with R’s C API within unwind_protect()
, then
any R errors will result in a jump that prevents your destructors from
running.
Nested unwind_protect()
Another issue that can arise has to do with nested calls to
unwind_protect()
. It is very hard (if not impossible) to
end up with invalidly nested unwind_protect()
calls when
using the typical cpp11 API, but you can manually create a scenario like
the following:
#include <cpp11.hpp>
[[cpp11::register]]
void test_nested() {
::unwind_protect([&] {
cpp11::unwind_protect([&] {
cpp11("oh no!");
Rf_error});
});
}
If you were to run test_nested()
from R, it would likely
crash or hang your R session due to the following chain of events:
-
test_nested()
sets up a try/catch to catch unwind exceptions - The outer
unwind_protect()
is called. It uses the C functionR_UnwindProtect()
to call its lambda function. - The inner
unwind_protect()
is called. It again usesR_UnwindProtect()
, this time to callRf_error()
. -
Rf_error()
performs alongjmp()
which is caught by the innerunwind_protect()
and promoted to an exception. - That exception is thrown, but because we are in the outer call to
R_UnwindProtect()
(a C function), we end up throwing that exception across C stack frames. This is undefined behavior, which is known to have caused R to crash on certain platforms.
You might think that you’d never do this, but the same scenario can
also occur with a combination of 1 call to unwind_protect()
combined with usage of the cpp11 API:
#include <cpp11.hpp>
[[cpp11::register]]
void test_hidden_nested() {
::unwind_protect([&] {
cpp11::stop("oh no!");
cpp11});
}
Because cpp11::stop()
(and most of the cpp11 API) uses
unwind_protect()
internally, we’ve indirectly ended up in a
nested unwind_protect()
scenario again.
In general, if you must use unwind_protect()
then you
must be very careful not to use any of the cpp11 API inside of the
unwind_protect()
call.
It is worth pointing out that calling out to an R function from cpp11
which then calls back into cpp11 is still safe, i.e. if the registered
version of the imaginary test_outer()
function below was
called from R, then that would work:
#include <cpp11.hpp>
[[cpp11::register]]
void test_inner() {
::stop("oh no!")
cpp11}
[[cpp11::register]]
void test_outer() {
auto fn = cpp11::package("mypackage")["test_inner"]
();
fn}
This might seem unsafe because cpp11::package()
uses
unwind_protect()
to call the R function for
test_inner()
, which then goes back into C++ to call
cpp11::stop()
, which itself uses
unwind_protect()
, so it seems like we are in a nested
scenario, but this scenario does actually work. It makes more sense if
we analyze it one step at a time:
- Call the R function for
test_outer()
- A try/catch is set up to catch unwind exceptions
- The C++ function for
test_outer()
is called -
cpp11::package()
usesunwind_protect()
to call the R function fortest_inner()
- Call the R function for
test_inner()
- A try/catch is set up to catch unwind exceptions (this is the key!)
- The C++ function for
test_inner()
is called -
cpp11::stop("oh no!")
is called, which usesunwind_protect()
to callRf_error()
, causing alongjmp()
, which is caught by thatunwind_protect()
and promoted to an exception. - That exception is thrown, but this time it is caught by the
try/catch set up by
test_inner()
as we entered it from the R side. This prevents that exception from crossing the C++ -> C boundary. - The try/catch calls
R_ContinueUnwind()
, whichlongjmp()
s again, and now theunwind_protect()
set up bycpp11::package()
catches that, and promotes it to an exception. - That exception is thrown and caught by the try/catch set up by
test_outer()
. - The try/catch calls
R_ContinueUnwind()
, whichlongjmp()
s again, and at this point we can safely let thelongjmp()
proceed to force an R error.
16. Ok but I really want to call
cpp11::unwind_protect()
manually
If you’ve read the above bullet and still feel like you need to call
unwind_protect()
, then you should keep in mind the
following when writing the function to unwind-protect:
- You shouldn’t create any C++ objects that have destructors.
- You shouldn’t use any parts of the cpp11 API that may call
unwind_protect()
. - You must be very careful not to call
unwind_protect()
in a nested manner.
In other words, if you only use plain-old-data types, are careful to
never throw exceptions, and only use R’s C API, then you can use
unwind_protect()
.
One place you may want to do this is when working with long character
vectors. Unfortunately, due to the way cpp11 must protect the individual
CHARSXP objects that make up a character vector, it can currently be
quite slow to use the cpp11 API for this. Consider this example of
extracting out individual elements with x[i]
vs using the
native R API:
#include <cpp11.hpp>
[[cpp11::register]]
::sexp test_extract_cpp11(cpp11::strings x) {
cpp11const R_xlen_t size = x.size();
for (R_xlen_t i = 0; i < size; ++i) {
(void) x[i];
}
return R_NilValue;
}
[[cpp11::register]]
::sexp test_extract_r_api(cpp11::strings x) {
cpp11const R_xlen_t size = x.size();
const SEXP data{x};
::unwind_protect([&] {
cpp11for (R_xlen_t i = 0; i < size; ++i) {
(void) STRING_ELT(data, i);
}
});
return R_NilValue;
}
set.seed(123)
x <- sample(letters, 1e6, replace = TRUE)
bench::mark(
test_extract_cpp11(x),
test_extract_r_api(x)
)
#> Warning: Some expressions had a GC in every iteration; so filtering is
#> disabled.
#> # A tibble: 2 × 6
#> expression min median `itr/sec` mem_alloc `gc/sec`
#> <bch:expr> <bch:tm> <bch:tm> <dbl> <bch:byt> <dbl>
#> 1 test_extract_cpp11(x) 39.86ms 42.88ms 22.4 0B 43.0
#> 2 test_extract_r_api(x) 2.16ms 2.17ms 459. 0B 0
We plan to improve on this in the future, but for now this is one of
the only places where we feel it is reasonable to call
unwind_protect()
manually.