From acc0ac84d9ddee95f4e38d7a9db2831a61d33205 Mon Sep 17 00:00:00 2001
From: Vladimir Tamara <vtamara@pasosdeJesus.org>
Date: Thu, 26 Mar 2020 13:19:32 -0500
Subject: [PATCH] Uses CXX17 if it can compile test program using timespec_get
 and TIME_UTC, otherwise by default uses CXX11. Fixes #115

---
 configure       | 40 ++++++++++++++++++++++++++++++++++++++--
 src/Makevars.in |  3 +--
 2 files changed, 39 insertions(+), 4 deletions(-)

diff --git a/configure b/configure
index 85c118c..28ccab3 100755
--- a/configure
+++ b/configure
@@ -3,7 +3,7 @@
 echo "Running configure script"
 
 # Find compiler
-CC=$("${R_HOME}"/bin/R CMD config CC)
+CC=`"${R_HOME}"/bin/R CMD config CC`
 
 # Detect whether -latomic is needed during linking. This is needed on some
 # platforms, notably ARM (Raspberry Pi).
@@ -21,8 +21,44 @@ else
 fi
 
 
+CXX=`"${R_HOME}"/bin/R CMD config CXX`
+CXX11STD=`"${R_HOME}"/bin/R CMD config CXX11STD`
+CXX11FLAGS=`"${R_HOME}"/bin/R CMD config CXX11FLAGS`
+CXX_STD=CXX11
+if [ "$CXX" != "" -a "$CXX11FLAGS" != "" -a "$CXX11STD" != "" ]; then
+        # Detect if it can use c++17
+        ptimespec='#include <cstdio>
+#include <ctime>
+// Example adapted from https://en.cppreference.com/w/cpp/chrono/c/timespec_get
+
+namespace std {
+
+        int main()
+        {
+                timespec ts;
+                timespec_get(&ts, TIME_UTC);
+                char buf[100];
+                strftime(buf, sizeof buf, "%D %T", gmtime(&ts.tv_sec));
+                printf("Current time: %s.%09ld UTC", buf, ts.tv_nsec);
+                return 0;
+        }
+
+}'
+        CXX17FLAGS=`"${R_HOME}"/bin/R CMD config CXX17FLAGS`
+        CXX17STD=`"${R_HOME}"/bin/R CMD config CXX17STD`
+        echo "$ptimespec" | ${CXX} -x c++ -c $CXX17FLAGS $CXX17STD - -o /dev/null > /dev/null 2>&1
+        if [ $? -eq 0 ]; then
+                # See https://github.com/r-lib/later/issues/115
+                echo "Using c++17."
+                CXX_STD=CXX17
+        else
+                echo "Cannot use c++17 using default configuration for c++11"
+        fi
+fi
+
+
 # Write to Makevars
-sed -e "s|@extra_pkg_libs@|$EXTRA_PKG_LIBS|" src/Makevars.in > src/Makevars
+sed -e "s|@extra_pkg_libs@|$EXTRA_PKG_LIBS|;s|@cxx_std@|$CXX_STD|" src/Makevars.in > src/Makevars
 
 # Success
 exit 0
diff --git a/src/Makevars.in b/src/Makevars.in
index f3d2a50..9227f2e 100644
--- a/src/Makevars.in
+++ b/src/Makevars.in
@@ -1,5 +1,4 @@
-# Use C++11 if available
-CXX_STD=CXX11
+CXX_STD=@cxx_std@
 
 PKG_CPPFLAGS = -pthread -DSTRICT_R_HEADERS
 PKG_LIBS = -pthread @extra_pkg_libs@