On 30 July 2017 at 19:49, Iñaki Úcar wrote:
| 2017-07-30 14:15 GMT+02:00 Dirk Eddelbuettel <e...@debian.org>:
| >
| > On 30 July 2017 at 02:38, Iñaki Úcar wrote:
| > | One last question: would you consider adding a cppXPtr() function to
| > | Rcpp? It would work as cppFunction(), but it would return a XPtr
| > | instead of the function. I have a working example if you are
| > | interested.
| >
| > "Possibly" -- API and functionality extensions are easier than changes; it
| > still adds extra documentation and testing.
| >
| > Can you detail what you are proposing some more, show an example etc pp?
|
| Of course. RcppDE would be a use case for this (and my simulator,
| another one). Currently, the user must provide not only the C++
| function, but also the adapter: a getter that instantiates an XPtr and
| pushes it to R space. It is not difficult, but 1) it's not so
| immediate nor intuitive, you need to instruct the user about this, and
| 2) it's repetitive and a nuisance.
|
| So my proposal is to automate this. A new cppXPtr() function would
| accept a single C++ function, just like cppFunction, but instead of
| exporting it, it would:
|
| - Detect the function signature. This is easy to do with some regexp,
| because it should be C-compatible and you won't have to deal with more
| complicated stuff like templates, etc. Example: "SEXP foo(int n,
| double l) { return rexp(n, l); }" would produce
|
| - name: foo
| - args: (int n, double l)
| - retv: SEXP
|
| - Construct and append a getter. For the example above,
|
| SEXP getXPtr() {
| typedef SEXP (*funcPtr)(int n, double l);
| return XPtr<funcPtr>(new funcPtr(&foo));
| }
|
| - Export the getter, call it, remove the getter from the environment
| and return the externalptr.
|
| An initial implementation, reusing code (omitted) from cppFunction,
| would be (please, find attached a complete example):
|
| cppXPtr <- function(...) { # same args as cppFunction
| # get signature
| func <- strsplit(code, "[[:space:]]*\\{")[[1]][[1]]
| func <- strsplit(func, "[[:space:]]*\\(")[[1]]
| args <- func[[2]]
| func <- strsplit(func, "[[:space:]]+")[[1]]
| name <- func[[length(func)]]
| retv <- func[seq_len(length(func)-1)]
|
| # process depends
| # process plugins
| # remainder of scaffolding
| # prepend scaffolding to code
|
| # append a getter
| code <- paste(c(
| code,
| "// [[Rcpp::export]]",
| "SEXP getXPtr() {",
| paste(" typedef", retv, "(*funcPtr)(", args, ";"),
| paste(" return XPtr<funcPtr>(new funcPtr(&", name, "));"),
| "}"), collapse="\n")
|
| # print the generated code if we are in verbose mode
| # source cpp into specified environment
|
| # verify that two functions were exported and return the XPtr
| if (length(exported$functions) == 0)
| stop("No function definition found")
| else if (length(exported$functions) > 2)
| stop("More than one function definition")
| else {
| ptr <- env$getXPtr()
| rm(list=exported$functions, envir = env)
| ptr
| }
| }
This might be most suitable for an add-on package, at least until all bugs
are ironed out. The use case and scope is more limited than for cppFunction().
Dirk
--
http://dirk.eddelbuettel.com | @eddelbuettel | e...@debian.org
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