RcppSpdlog: Bundling of spdlog for use from R and Rcpp

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The spdlog library is a widely-used and very capable header-only C++ library for logging. This package includes its headers as an R package to permit other R packages to deploy it via a simple LinkingTo: RcppSpdlog as described in Section 1.1.3 of WRE.


A simple first example, following the upstream examples:

edd@rob:~$ Rscript -e 'Rcpp::sourceCpp("inst/examples/exampleOne.cpp")'

R> exampleOne()
[07:45:57.168673] [I] [thread 1500593] Welcome to spdlog!
[07:45:57.168704] [E] [thread 1500593] Some error message with arg: 1
[07:45:57.168707] [W] [thread 1500593] Easy padding in numbers like 00000012
[07:45:57.168710] [C] [thread 1500593] Support for int: 42;  hex: 2a;  oct: 52; bin: 101010
[07:45:57.168728] [I] [thread 1500593] Support for floats 1.23
[07:45:57.168731] [I] [thread 1500593] Positional args are supported too..
[07:45:57.168734] [I] [thread 1500593] left aligned                
[07:45:57.168737] [D] [thread 1500593] This message should be displayed..

This logs the hour, minute, second, microsecond followed by a one-char code for info, error, warning or critical followed by the thread id and the actual loggable message. The code, apart from the included headers and more, is simply

// [[Rcpp::export]]
void exampleOne() {

  // change log pattern (changed from [%H:%M:%S %z] [%n] [%^---%L---%$] )
  spdlog::set_pattern("[%H:%M:%S.%f] [%L] [thread %t] %v");

  spdlog::info("Welcome to spdlog!");
  spdlog::error("Some error message with arg: {}", 1);

  spdlog::warn("Easy padding in numbers like {:08d}", 12);
  spdlog::critical("Support for int: {0:d};  hex: {0:x};  oct: {0:o}; bin: {0:b}", 42);
  spdlog::info("Support for floats {:03.2f}", 1.23456);
  spdlog::info("Positional args are {1} {0}..", "too", "supported");
  spdlog::info("{:<30}", "left aligned");

  spdlog::set_level(spdlog::level::debug); // Set global log level to debug
  spdlog::debug("This message should be displayed..");

  // Compile time log levels
  // define SPDLOG_ACTIVE_LEVEL to desired level
  SPDLOG_TRACE("Some trace message with param {}", {});
  SPDLOG_DEBUG("Some debug message");


Many other customizations are possible, see the spdlog wiki.

Note that using spdlog examples directly may well trigger warning from R during package checking as stdout and/or stderr may be used. See the included example function described in the next section which uses a derived class to pass logging output explicitly to the R input/output stream as per the R coding requirements, see Section 1.3.1 (and others) of WRE.

Use in R Packages and Warnings

As shipped, both spdlog and the embedded fmt use stdout and stderr in ways that may make it non-trivial to fully replace them with R input/output as required by Section 1.3.1 (and others) of WRE.

However, based on some initial trials and some excellent help from upstream we have defined a specific sink for R in the header rcpp_sink.h, corrected one stderr use and added one #define. That combination now passes as can be seen in checks of the package RcppSpdlog and the included function exampleRsink() whose complete source code is included here:

// this define is important to not include another logger pulling in stdout

// this portmanteau include also defines the r_sink we use below, and which
// diverts all logging to R via the Rcpp::Rcout replacement for std::cout
#include <RcppSpdlog>

//' spdlog Example using a sink for R
//' A simple example invoking a derived R/Rcpp logger.
//' Note that this no longer triggers R warnings thanks to excellent help by
//' Gabi Melman.
//' @return None
//' @examples
//' exampleRsink()
// [[Rcpp::export]]
void exampleRsink() {

    std::string logname = "fromR";                          // fix a name for this logger
    auto sp = spdlog::get(logname);                         // retrieve existing one
    if (sp == nullptr) sp = spdlog::r_sink_mt(logname);     // or create new one if needed

    // change log pattern (changed from [%H:%M:%S %z] [%n] [%^---%L---%$] )
    sp->set_pattern("[%H:%M:%S.%f] [%n] [%^%L%$] [thread %t] %v");

    sp->info("Welcome to spdlog!");
    sp->error("Some error message with arg: {}", 1);

    sp->warn("Easy padding in numbers like {:08d}", 12);
    sp->critical("Support for int: {0:d};  hex: {0:x};  oct: {0:o}; bin: {0:b}", 42);
    sp->info("Support for floats {:03.2f}", 1.23456);
    sp->info("Positional args are {1} {0}..", "too", "supported");
    sp->info("{:<30}", "left aligned");


Note that it is deliberately similar in use to the example above. A new instance of the logger is instantiated as a shared pointer sp to a spdlog object.

We may make additional package features available in the future.


Gabi Melman is the main author of spdlog.

Victor Zverovich is the main author of the embedded fmt library.

Dirk Eddelbuettel is author of this package and the R integration.


spdlog and fmt are under the MIT license.

RcppSpdlog is released under the GNU GPL, version 2 or later, just like R and Rcpp.

Initially created: Wed Aug 19 08:01:13 CDT 2020
Last modified: Wed Sep 16 19:08:56 CDT 2020