Implement RFC68: C++11

[Kurt Schwehr]

A place for discussion of implementing RFC 68: C++11 Compilation Mode.

[Kurt Schwehr]

I didn't like this first thing I tried. I started with just trying to remove the --with_cpp11 flag. I think it will be better to change --with_cpp11 to --with_cpp14 and have the default case enforce C++11.

To enforce C++11 (and also allow C++14), I'll add an #if in cpl_port.h that does a #error if __cplusplus is less than 201103.

I am planning to test this a pull requests so that we can work through the travis-ci builds. I will also attempt the windows builds, but I could use help.

gdal/configure.ac

@@ -123 +123 @@
 
 AC_LANG_POP([C++])
 
-CXX11_SUPPORT=no
-AC_ARG_WITH([cpp11],
-            AS_HELP_STRING([--without-cpp11],
-               [Disable C++11 compiler options]),,)
-
-AC_MSG_CHECKING([if use C++11 compiler options])
-if test "$with_cpp11" = "yes" -o "$with_cpp11" = "" ; then
-    dnl Check C++ compiler compatibility with C++11
-    AC_MSG_CHECKING([C++ compiler compatibility with C++11])
-    AX_CXX_COMPILE_STDCXX_11([ext],[optional])
-
-    if [test "$HAVE_CXX11" = "1"]; then
-        CXX11_SUPPORT=yes
-    fi
-    AC_MSG_RESULT([enabled])
-else
-    AC_MSG_RESULT([disabled by user])
-fi
-AC_SUBST(CXX11_SUPPORT,$CXX11_SUPPORT)
+AC_MSG_CHECKING([C++ compiler compatibility with C++11])
+AX_CXX_COMPILE_STDCXX_11([ext],[mandatory])
 
-if [test "$CXX11_SUPPORT" = "yes"]; then
-    dnl Available in GCC 5.1
-    AC_LANG_PUSH([C++])
-        AX_CHECK_COMPILE_FLAG([-Wsuggest-override], [CXX_WFLAGS="$CXX_WFLAGS -Wsuggest-override"],,[$ERROR_ON_UNKNOWN_OPTIONS])
-    AC_LANG_POP([C++])
-fi
+dnl Available in GCC 5.1
+AC_LANG_PUSH([C++])
+    AX_CHECK_COMPILE_FLAG([-Wsuggest-override], [CXX_WFLAGS="$CXX_WFLAGS -Wsuggest-override"],,[$ERROR_ON_UNKNOWN_OPTIONS])
+AC_LANG_POP([C++])
 
 dnl Enable -Wimplicit-fallthrough only if C++11 is enabled since CPL_FALLTHROUGH is only active then
 AC_LANG_PUSH([C++])
@@ -156 +137 @@
 CXXFLAGS="$CXXFLAGS $ERROR_ON_UNKNOWN_OPTIONS -Wimplicit-fallthrough"
 AC_MSG_CHECKING([if -Wimplicit-fallthrough can be enabled])
 AC_COMPILE_IFELSE([AC_LANG_PROGRAM(
-  [[#if !(__cplusplus >= 201103L && ( (defined(__clang__) && (__clang_major__ > 3 || (__clang_major__ == 3 && __clang_minor__ >=7))) || ((defined(__GNUC__) && __GNUC__ >= 7)) ) )
+  [[#if !(( (defined(__clang__) && (__clang_major__ > 3 || (__clang_major__ == 3 && __clang_minor__ >=7))) || ((defined(__GNUC__) && __GNUC__ >= 7)) ) )
     #error "not (C++11 and (clang >= 3.7 or gcc >= 7))"
     #endif]])],
   [CXX_WFLAGS="$CXX_WFLAGS -Wimplicit-fallthrough"]
@@ -5566 +5547 @@
 LOC_MSG([  Installation directory:    ${prefix}])
 LOC_MSG([  C compiler:                ${CC} ${CFLAGS}])
 LOC_MSG([  C++ compiler:              ${CXX} ${CXXFLAGS}])
-LOC_MSG([  C++11 support:             ${CXX11_SUPPORT}])
 LOC_MSG()
 LOC_MSG([  LIBTOOL support:           ${with_libtool}])
 LOC_MSG()

[Kurt Schwehr]

Here is what mloskot did with GEOS:

• ​https://github.com/OSGeo/geos/commit/1e2b2a2600c4ee31378842cc3c46d61cfad9efce
• ​https://github.com/OSGeo/geos/commit/4c9b08743a31f3ecf3fee742cbaffcb69e00d2f7

configure.{ac,in}:

AX_CXX_COMPILE_STDCXX_11([noext],[mandatory])

Or via trac:

• ​https://trac.osgeo.org/geos/changeset/6932138816c720c5b90dd7490541274e40fb50f1/git
• ​https://trac.osgeo.org/geos/changeset/4c9b08743a31f3ecf3fee742cbaffcb69e00d2f7/git
• ​https://trac.osgeo.org/geos/changeset/b507095ff861fbe8642cd65660bf530e674efb2f/git
• ​https://trac.osgeo.org/geos/changeset/1e2b2a2600c4ee31378842cc3c46d61cfad9efce/git

unique_ptr:

• ​https://trac.osgeo.org/geos/changeset/8193383d7020b369205c79ba9021e8b1774a84ca/git

[Mateusz Łoskot]

Kurt, FYI, my Autoconf is a bit rusty, so those changes in GEOS might not be optimal.

[Kurt Schwehr]

2nd attempt still has issues, but it feels like it is on track. Need to figure out how to get python to use the correct compile flag.

​https://github.com/OSGeo/gdal/pull/243/commits/9c2fa9b7939defa237c57ff0dde56f59fc98e7ed

Created a pull request to start testing with travis to see where things are at. I bet all the builds will fail. Most failed pretty quickly

• ​https://github.com/OSGeo/gdal/pull/243
• ​https://travis-ci.org/OSGeo/gdal/builds/282580217?

trusty_clang:

/usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/cstdio:120:11: error: no member named 'gets' in the global namespace
  using ::gets;
        ~~^

mingw mingw64 python3 android:

configure: error: *** A compiler with support for C++11 language features is required.

bigendian:

In file included from gdalallregister.cpp:30:0:
/home/travis/build/OSGeo/gdal/gdal/gcore/gdal_priv.h:532:21: error: 'virtual char** GDALDataset::GetMetadata(const char*)' can be marked override [-Werror=suggest-override]
     virtual char ** GetMetadata(const char * pszDomain = "") CPL_OVERRIDE;

gcc48_stdcpp11

gcc52_stdcpp14

precise32

[Kurt Schwehr]

trusty clang:

/usr/lib/gcc/x86_64-linux-gnu/4.8/../../../../include/c++/4.8/cstdio:120:11: error: no member named 'gets' in the global namespace
  using ::gets;
        ~~^

​https://stackoverflow.com/questions/24342312/clang-seems-to-use-the-gcc-libraries

You need to install libc++ and make clang use it with -stdlib=libc++

[Even Rouault]

C++11 requirement and related changes now committed in trunk