spdl: A consistent C++ and R interface to spdlog

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The RcppSpdlog repository (and CRAN package) provides access to the wonderful spdlog library along with the fmt library.

This permits use in C++ extensions for R by offering the spdlog (header-only) library along with the (header-only) fmt library (by using LinkingTo as described in Section 1.1.3 of WRE). More recently, RcppSpdlog was extended so that it provides a set of key functions directly for use by other R functions (as described in Section 5.4.3 of WRE).

However, now the use of, say, a debug logging message from C++ looks like

// in C++
spdlog::debug("Text with {} text {} which is {}", "auto", "expansion", 42);

whereas in R, given the RcppSpdlog package and namespace, it looks like this (if we use sprintf() to assemble the message)

# in R
RcppSpdlog::log_debug(sprintf("Text with %s text %s which is %s", "auto", "expansion", 42L)

and that irked us. Enter this package! By owning the spld namespace (in R) and an easily overlayed namespace in C++ of the same name we can do

// in C++
spdl::debug("Text with {} text {} which is {}", "auto", "expansion", 42);

as well as (still using sprintf())

# in R
spdl::debug(sprintf("Text with %s text %s which is %s", "auto", "expansion", 42L))

which is much better as it avoids context switching. Better still, with the internal formatter we can write the same format string as in C++ and not worry about format details:

# in R
spdl::debug("Text with {} text {} which is {}", "auto", "expansion", 42L)


We use a simple mechanism in which all R arguments are passed through format() by default to render strings, and then pass a single vector of strings argument through the restrictive C language Foreign Function Interface to RcppSpdlog where it can be passed to the C++ layer available there.

This also means we use the fmt library in both languages as the formatter. We prefer this is over other string-interpolating libraries in R which are similar but subtly different. Should their use be desired, they can of course be used: the default call to any of the loggers is just a single-argument call with a text variable so users are free to expand strings as they please. Anything starting from paste and sprintf works.

As of release 0.0.2, we also expose helpers spdl::fmt() (to format) and spdl::cat() (to display).


Note that because the package uses functions names also present in the base R packages (namely cat, debug, drop, trace) we do not recommend loading the package. Instead, call it with the explicit prefix as e.g. spdl::debug("Some message {}", foo). As a selective importFrom one can always do importFrom("spdl", "setup") combined with the explicit pre-fix use.


Dirk Eddelbuettel


spdl is released under the GNU GPL, version 2 or later, just like R itself.