Problem: no automated way to enforce C++98 compatibility

Solution: if the compiler supports it, pass C++98-compat flags.
Currently Clang supports this flag but GCC does not.

Add a new flag to enable it, as building with C++98-compat but also
with -std=gnu++11 will cause a lot of warnings due to the backward
compat ifdefs.

Add a CI job to run it and ensure we don't break compatibility.

.travis.yml

@@ -111,6 +111,19 @@ matrix:
     os: linux
   - env: BUILD_TYPE=default POLLER=select
     os: linux
+  - env: CXX=clang++ BUILD_TYPE=default CURVE=libsodium GSSAPI=enabled PGM=enabled NORM=enabled FORCE_98=enabled
+    os: linux
+    compiler: clang
+    addons:
+      apt:
+        sources:
+        - sourceline: 'deb http://download.opensuse.org/repositories/network:/messaging:/zeromq:/git-stable/xUbuntu_14.04/ ./'
+          key_url: 'http://download.opensuse.org/repositories/network:/messaging:/zeromq:/git-stable/xUbuntu_14.04/Release.key'
+        packages:
+        - libkrb5-dev
+        - libnorm-dev
+        - libpgm-dev
+        - libsodium-dev
     
 sudo: false
 

NEWS

@@ -55,6 +55,11 @@
   different internal implementations of the trie algorithm used to track
   subscriptions. Requires a compiler that supports C++11.
 
+* New autoconf flag "--enable-force-CXX98-compat" which will force -std=gnu++98
+  and, if the compiler supports them (clang++ at the moment), it will also add
+  -Wc++98-compat -Wc++98-compat-pedantic so that compatibility with C++98 can
+  be tested.
+
 * Many, many coding style, duplication and static analysis improvements.
 
 * Many, many improvements to the CMake build system, especially on Windows.

ci_build.sh

@@ -62,6 +62,10 @@ if [ $BUILD_TYPE == "default" ]; then
         CONFIG_OPTS+=("--enable-drafts=yes")
     fi
 
+    if [ -n "$FORCE_98" ] && [ "$FORCE_98" == "enabled" ]; then
+        CONFIG_OPTS+=("--enable-force-CXX98-compat=yes")
+    fi
+
     # Build and check this project
     (
         ./autogen.sh &&

configure.ac

@@ -58,7 +58,6 @@ ZMQ_ORIG_CXXFLAGS="${CXXFLAGS:-none}"
 AC_PROG_CC
 AX_CHECK_COMPILE_FLAG([-std=gnu11], [CFLAGS+=" -std=gnu11"], [AC_PROG_CC_C99])
 AC_PROG_CXX
-AX_CXX_COMPILE_STDCXX_11([ext], [optional])
 AX_CODE_COVERAGE
 AM_PROG_CC_C_O
 AC_PROG_SED
@@ -76,6 +75,17 @@ AC_LIBTOOL_WIN32_DLL
 AC_PROG_LIBTOOL
 AX_VALGRIND_CHECK
 
+AC_ARG_ENABLE([force-CXX98-compat],
+    [AS_HELP_STRING([--enable-force-CXX98-compat], [force C++98 build [default=disabled]])])
+if test "x$enable_force_CXX98_compat" = "xyes"; then
+    AC_LANG_PUSH([C++])
+    AX_CHECK_COMPILE_FLAG([-std=gnu++98], [CXXFLAGS+=" -std=gnu++98"], [])
+    AX_CHECK_COMPILE_FLAG([-Wc++98-compat -Wc++98-compat-pedantic], [CXXFLAGS+=" -Wc++98-compat"], [])
+    AC_LANG_POP([C++])
+else
+    AX_CXX_COMPILE_STDCXX_11([ext], [optional])
+fi
+
 # Check whether to build a with debug symbols
 LIBZMQ_CHECK_ENABLE_DEBUG