core: add option to force compiling C++ as C++11

Many C++ libraries like boost and log4cplus use defines to be compatible with both C++03 and C++11 standards. This causes ABI incompatibilities between a library built with C++03 and an application built with C++11. To avoid this, one has to built libraries as C++11 as well.

This patch introduces the BR2_ENABLE_CPP11 configuration option that forces passing --std=c++11 to the compiler while building C++ target libraries.

Signed-off-by Alexey Galakhov <[email protected]>

---
 Config.in           | 17 +++++++++++++++++
 package/Makefile.in | 10 +++++++++-
 2 files changed, 26 insertions(+), 1 deletion(-)

Config.in +17 -0

index d795361..1ee2ca3 100644
@@ -533,6 +533,23 @@ config BR2_ENABLE_SSP
 comment "enabling Stack Smashing Protection requires support in the toolchain"
 	depends on !BR2_TOOLCHAIN_HAS_SSP
 
+config BR2_ENABLE_CPP11
+	bool "force C++11 when building libraries"
+	default n
+	depends on BR2_INSTALL_LIBSTDCPP
+	depends on BR2_TOOLCHAIN_GCC_AT_LEAST_4_3
+	help
+	  Force building of C++ libraries in C++11 mode.
+
+	  Some C++ libraries (i.e. boost) behave differently if compiled with
+	  and without --std=c++11 flag. They define classes differently. This
+	  causes linker errors while trying to use such a library in a C++11
+	  project.
+
+	  This option forces building of all such libraries in C++11 mode.
+	  Enable it if you encounter C++11-related linker errors while building
+	  your own or some 3rd-party software against buildroot libraries.
+
 choice
 	bool "libraries"
 	default BR2_SHARED_LIBS if BR2_BINFMT_SUPPORTS_SHARED

package/Makefile.in +9 -1

index 85008bb..0cca97e 100644
@@ -135,9 +135,17 @@ ifeq ($(BR2_DEBUG_3),y)
 TARGET_DEBUGGING = -g3
 endif
 
+ifeq ($(BR2_ENABLE_CPP11),y)
+ifeq ($(BR2_TOOLCHAIN_GCC_AT_LEAST_4_7),y)
+TARGET_CXXLANGUAGE = --std=c++11
+else
+TARGET_CXXLANGUAGE = --std=c++0x
+endif
+endif
+
 TARGET_CPPFLAGS += -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64
 TARGET_CFLAGS = $(TARGET_CPPFLAGS) $(TARGET_ABI) $(TARGET_OPTIMIZATION) $(TARGET_DEBUGGING)
-TARGET_CXXFLAGS = $(TARGET_CFLAGS)
+TARGET_CXXFLAGS = $(TARGET_CFLAGS) $(TARGET_CXXLANGUAGE)
 TARGET_LDFLAGS = $(call qstrip,$(BR2_TARGET_LDFLAGS))
 
 ifeq ($(BR2_BINFMT_FLAT),y)

Quoted 11 lines · Alexey Galakhov

Many C++ libraries like boost and log4cplus use defines to be compatible with both C++03 and C++11 standards. This causes ABI incompatibilities between a library built with C++03 and an application built with C++11. To avoid this, one has to built libraries as C++11 as well.

This patch introduces the BR2_ENABLE_CPP11 configuration option that forces passing --std=c++11 to the compiler while building C++ target libraries.

Signed-off-by Alexey Galakhov <[email protected]>

Hi Alexey,

I've been mulling about this for some time and I'm not sure if we
really want this. First of all, as I wrote before, it should probably be done automatically somehow instead of being user-selectable. However, I'm also afraid that some packages will fail to build with --std=c++11. Since we will not enable this option in the autobuilders, we will not notice such problems.

Do you know of other packages (other than boost) that expose this
problem? Perhaps instead we should add an option to boost only?

Many C++ libraries like boost and log4cplus use defines to be compatible with both C++03 and C++11 standards. This causes ABI incompatibilities between a library built with C++03 and an application built with C++11. To avoid this, one has to built libraries as C++11 as well.

This patch introduces the BR2_ENABLE_CPP11 configuration option that forces passing --std=c++11 to the compiler while building C++ target libraries.

Signed-off-by Alexey Galakhov <[email protected]>