[Rcpp-devel] How to handle std::cout/std::cerr in shared libraries

[Watal M. Iwasaki]

Dear Dirk,

Thank you for the prompt response. Good to know there is no easy way. I
have made up my mind to change the library code as you suggested. But I
don't like preprocessor macro; therefore, the problem here was solved by
moving/hiding `std::cout.rdbuf()` part into the library as a function that
takes a streambuf pointer, and just calling it from Rcpp side. Now, output
is properly sent to R console, R CMD check complains nothing, and the
library still remains free from R/Rcpp code. Thanks again.

Best,
Watal

On Fri, Oct 26, 2018 at 9:29 PM Dirk Eddelbuettel <edd at debian.org> wrote:

>
> On 26 October 2018 at 16:31, Watal M. Iwasaki wrote:
> | I have been working on an R interface package to a shared library written
> | in C++. It works almost perfectly, but the output to std::cout is not
> shown
> | on R console (on Linux). Of course Rcpp::Rcout should be used instead of
> | std::cout. But in this case I want to let the shared library be pure C++.
> | So I tried switching the stream buffers in the Rcpp side:
> | ```
> | Rcpp::CharacterVector run(const std::vector<std::string>& args) {
> |     std::streambuf* obuf = std::cout.rdbuf(Rcpp::Rcout.rdbuf());
> |     mylib::Simulation simulation(args);
> |     simulation.run(); // std::cout is used in here
> |     std::cout.rdbuf(obuf);
> |     return "something";
> | }
> | ```
> |
> | It (seemingly) works! But now `devtools::check()` produces 1 note:
> | ```
> | ❯ checking compiled code ... NOTE
> |   File ‘mylib/libs/mylib.so’:
> |     Found ‘__ZNSt3__14coutE’, possibly from ‘std::cout’ (C++)
> |       Object: ‘run.o’
> |
> |   Compiled code should not call entry points which might terminate R nor
> |   write to stdout/stderr instead of to the console, nor use Fortran I/O
> |   nor system RNGs.
> | ```
> |
> | Can I suppress this note? or should I just ignore it? any other good
> | solution?
>
> You need to change it. There is no automatic fix.
>
> [ Longer answer: See eg what we do in RcppArmadillo where we #define a
> device
> which for builds that we do from R fills in Rcpp::Rcout and otherwise
> defaults to std::cout.  But the essence is the same: _You_ need to change
> that library code to conform.  Or keep the library outside the R package
> but
> then you have an external dependency. ]
>
> Dirk
>
> --
> http://dirk.eddelbuettel.com | @eddelbuettel | edd at debian.org
>


-- 
Watal M. Iwasaki / 岩嵜 航
SOKENDAI, The Graduate University for Advanced Studies,
Hayama, Kanagawa 240-0193, Japan
+81-46-858-1576
https://heavywatal.github.io/
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