Call Rcpp c++ function from R code in the same package

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I want to build an Rcpp package with C++ and R code. With the C++ code only (R code removed), everything compiles and works nicely and after building and loading the exported functions are callable as expected as 

<packageName>::<functionName>()

However, when including the R code with the call to the C++ code, I get an object not found error when building the package:

R CMD build <packageName>
[...]
** R
** byte-compile and prepare package for lazy loading
Error in <functionName>() :
  object '_<packageName>_<functionName>' not found
Error: unable to load R code in package '<packageName>'

The error happens after the C++ code is successfully compiled. 

<functionName>()

is mapped to 

_<packageName>_<functionName>

in R/RcppExports.R as usual, but it does not appear that it can be loaded by the R code at build time.

The structure of the package is as usual:

<packageName>
├── DESCRIPTION
├── man
│   ├── <functionName>.rd
│   └── <packageName>-package.rd
├── NAMESPACE
├── R
│   ├── <RCodeFileName>.R
│   └── RcppExports.R   
├── README.md
└── src
    ├── <C++CodeFileName>.cpp
    ├── <C++CodeFileName>.o
    ├── <packageName>.so
    ├── RCppExports.cpp
    ├── RCppExports.o
    └── symbols.rds

where the .o and .so files are produced when compiling the package.

The R file calls the function from the C++ file directly:

[other stuff]
[...]
<functionName>()

NAMESPACE file is also as usual:

useDynLib(<packageName>, .registration=TRUE)
importFrom(Rcpp, evalCpp)
exportPattern("^[[:alpha:]]+")

This seems like a very simple and straight forward problem (calling C++ code from R code in an Rcpp package). Nevertheless, I seem to be unable to find any indication of how this should be done.

Edit (Apr 8 2020): build vs. INSTALL

The problem will occur in every case for 

R CMD INSTALL <packageName>_<version>.tar.gz

If the included help file 

man/<packageName>-package.Rd

is built so that it forces installing the package to process help pages the error will already occur in 

R CMD build <packageName>

as described above.

Edit (Apr 8 2020): Steps to reproduce with Rcpp.package.skeleton() (on Unix-like systems anyway)

Rscript -e 'Rcpp::Rcpp.package.skeleton("demo20200408")'
echo 'rcpp_hello_world()' > errorDemo/R/example.R
R CMD INSTALL errorDemo
r-package rcpp r
1 Answers

You may want to slow dowm. The Rcpp package itself comes with a demo package generator via the Rcpp.package.skeleton() function. Run it!

The compare piece by piece to what you have.

A second generator is e.g. built into RStudio and available under the File -> New Project -> New directory -> Package with Rcpp menu options.

Otherwise, hard to tell. Did you do anything funky to you function name? Because these are mapped to R and C++ you have restrictions from both. I.e. you cannot use a dot (as that would be a class method notation in C++).

Lastly, even when unexported the C++ function should be available from the installed and loaded package via three colons, i.e. mypkg:::myFun().

Lastly, quick demo:

Create it 
edd@rob:/tmp$ Rscript -e 'Rcpp::Rcpp.package.skeleton("demo20200408")'
Creating directories ...
Creating DESCRIPTION ...
Creating NAMESPACE ...
Creating Read-and-delete-me ...
Saving functions and data ...
Making help files ...
Done.
Further steps are described in './demo20200408/Read-and-delete-me'.

Adding Rcpp settings
 >> added Imports: Rcpp
 >> added LinkingTo: Rcpp
 >> added useDynLib directive to NAMESPACE
 >> added importFrom(Rcpp, evalCpp) directive to NAMESPACE
 >> added example src file using Rcpp attributes
 >> added Rd file for rcpp_hello_world
 >> compiled Rcpp attributes 
edd@rob:/tmp$
Install it 
edd@rob:/tmp$ R CMD INSTALL demo20200408 
* installing to library ‘/usr/local/lib/R/site-library’
* installing *source* package ‘demo20200408’ ...
** using staged installation
** libs
ccache g++ -I"/usr/share/R/include" -DNDEBUG  -I"/usr/local/lib/R/site-library/Rcpp/include"   -fpic  -g -O3 -Wall -pipe -pedantic  -c RcppExports.cpp -o RcppExports.o
ccache g++ -I"/usr/share/R/include" -DNDEBUG  -I"/usr/local/lib/R/site-library/Rcpp/include"   -fpic  -g -O3 -Wall -pipe -pedantic  -c rcpp_hello_world.cpp -o rcpp_hello_world.o
ccache g++ -Wl,-S -shared -L/usr/lib/R/lib -Wl,-Bsymbolic-functions -Wl,-z,relro -o demo20200408.so RcppExports.o rcpp_hello_world.o -L/usr/lib/R/lib -lR
installing to /usr/local/lib/R/site-library/00LOCK-demo20200408/00new/demo20200408/libs
** R
** byte-compile and prepare package for lazy loading
** help
*** installing help indices
** building package indices
** testing if installed package can be loaded from temporary location
** checking absolute paths in shared objects and dynamic libraries
** testing if installed package can be loaded from final location
** testing if installed package keeps a record of temporary installation path
* DONE (demo20200408)
edd@rob:/tmp$
Run it 
edd@rob:/tmp$ Rscript -e 'library(demo20200408); rcpp_hello_world()'
[[1]]
[1] "foo" "bar"

[[2]]
[1] 0 1

edd@rob:/tmp$
And add an R function and call it too 
edd@rob:/tmp$ echo 'r_hello_world <- function() cat("hi there\n")' > demo20200408/R/foo.R
edd@rob:/tmp$ R CMD INSTALL demo20200408 
* installing to library ‘/usr/local/lib/R/site-library’
* installing *source* package ‘demo20200408’ ...
** using staged installation
** libs
make: Nothing to be done for 'all'.
installing to /usr/local/lib/R/site-library/00LOCK-demo20200408/00new/demo20200408/libs
** R
** byte-compile and prepare package for lazy loading
** help
*** installing help indices
** building package indices
** testing if installed package can be loaded from temporary location
** checking absolute paths in shared objects and dynamic libraries
** testing if installed package can be loaded from final location
** testing if installed package keeps a record of temporary installation path
* DONE (demo20200408)
edd@rob:/tmp$ Rscript -e 'library(demo20200408); r_hello_world()'
hi there
edd@rob:/tmp$
Still no issue despite OP's claims

In the most recent comment below it is claimed that the generated function cannot be called. That is false.

edd@rob:/tmp$ editor demo20200408/R/foo.R    # subst. fave editor here
edd@rob:/tmp$ cat demo20200408/R/foo.R 
r_hello_world <- function() {
        cat("hi there\n")
        ignored <- rcpp_hello_world()
        NULL
}
edd@rob:/tmp$ R CMD INSTALL demo20200408 
* installing to library ‘/usr/local/lib/R/site-library’
* installing *source* package ‘demo20200408’ ...
** using staged installation
** libs
make: Nothing to be done for 'all'.
installing to /usr/local/lib/R/site-library/00LOCK-demo20200408/00new/demo20200408/libs
** R
** byte-compile and prepare package for lazy loading
** help
*** installing help indices
** building package indices
** testing if installed package can be loaded from temporary location
** checking absolute paths in shared objects and dynamic libraries
** testing if installed package can be loaded from final location
** testing if installed package keeps a record of temporary installation path
* DONE (demo20200408)
edd@rob:/tmp$ Rscript -e 'library(demo20200408); r_hello_world()'
hi there
NULL
edd@rob:/tmp$
user_1433050
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