`write_compiler_detection_header` generates for the `cxx_static_assert` feature and the gcc compiler something like this: ```c++ # if (__GNUC__ * 100 + __GNUC_MINOR__) >= 404 && (__cplusplus >= 201103L || (defined(__GXX_EXPERIMENTAL_CXX0X__) && __GXX_EXPERIMENTAL_CXX0X__)) # define FOO_COMPILER_CXX_STATIC_ASSERT 1 # else # define FOO_COMPILER_CXX_STATIC_ASSERT 0 # endif ``` [...] ```c++ # if FOO_COMPILER_CXX_STATIC_ASSERT # define FOO_STATIC_ASSERT(X) static_assert(X, #X) # define FOO_STATIC_ASSERT_MSG(X, MSG) static_assert(X, MSG) # else template<bool> struct FOOStaticAssert; template<> struct FOOStaticAssert<true>{}; # define FOO_STATIC_ASSERT(X) sizeof(FOOStaticAssert<X>) # define FOO_STATIC_ASSERT_MSG(X, MSG) sizeof(FOOStaticAssert<X>) # endif ``` This macros, that works everywhere in c++11 will not work everywhere in c++98. For example: ```c++ #include "foo.h" template <int val> struct data_structure { FOO_STATIC_ASSERT_MSG((val > 0), "val should be greater than 0"); }; int main() { #ifndef FAIL_ASSERT data_structure<42> ds_ok; #else data_structure<-5> ds_not_ok; #endif } ``` In c++11: ``` $ g++ -std=c++11 -DFAIL_ASSERT=0 -o /tmp/a.out /tmp/foo.cpp # compiles as expected $ g++ -std=c++11 -DFAIL_ASSERT=1 -o /tmp/a.out /tmp/foo.cpp # fails as expected /tmp/foo.cpp: In instantiation of ‘struct data_structure<-5>’: /tmp/foo.cpp:32:24: required from here /tmp/foo.cpp:10:43: error: static assertion failed: val should be greater than 0 # define FOO_STATIC_ASSERT_MSG(X, MSG) static_assert(X, MSG) ^ /tmp/foo.cpp:23:5: note: in expansion of macro ‘FOO_STATIC_ASSERT_MSG’ FOO_STATIC_ASSERT_MSG((val > 0), "val should be greater than 0"); ^ ``` In c++98: ``` $ g++ -std=c++98 -DFAIL_ASSERT=0 -o /tmp/a.out /tmp/foo.cpp # fails!! /tmp/foo.cpp:15:43: error: expected unqualified-id before ‘sizeof’ # define FOO_STATIC_ASSERT_MSG(X, MSG) sizeof(FOOStaticAssert<X>) ^ /tmp/foo.cpp:23:5: note: in expansion of macro ‘FOO_STATIC_ASSERT_MSG’ FOO_STATIC_ASSERT_MSG((val > 0), "val should be greater than 0"); ^ ``` I don't know if this behaviour is expected, or if I'm doing something wrong but I cannot find documentation about this anywhere. Since this macro generates code that is not portable, I'd rather have it defined empty in c++98.