Can #if conditional areas cross include file boundaries?

Question

In the MSDN (https://msdn.microsoft.com/en-us/library/ew2hz0yd.aspx) I see the following:

All conditional-compilation directives, such as #if and #ifdef, must be matched with closing #endif directives prior to the end of file; otherwise, an error message is generated. When conditional-compilation directives are contained in include files, they must satisfy the same conditions: There must be no unmatched conditional-compilation directives at the end of the include file.

Well, simple and clear. A the same time I cannot find something like that in the C++11 standard. My question is this legal limitation?

I fully understand that splitting conditional compilation over several #include layers is not a good idea and should be avoided.

Does anybody know how other compilers (GCC, CLANG) handle this case? Maybe this was discussed somewhere?

Answer 1

#if FOO
#include "hashif.h"


extern "C" int printf(const char* fmt, ...);

int main()
{
    printf("Hello, World\n");
}

and hashif.h contains this:

#define BAR 1
#else
#define BAR 2
#endif

then clang++ gives an error.

hashif.cpp:1:2: error: unterminated conditional directive
#if FOO  
^
1 error generated.

Edit: And g++ and cpp all behave the same way.

Exact output from:

$ cpp hashif.cpp -DFOO=1
# 1 "hashif.cpp"
# 1 "<built-in>"
# 1 "<command-line>"
# 1 "/usr/include/stdc-predef.h" 1 3 4
# 1 "<command-line>" 2
# 1 "hashif.cpp"

# 1 "hashif.h" 1
In file included from hashif.cpp:2:0:
hashif.h:2:2: error: #else without #if
 #else
  ^
hashif.h:3:0: warning: "BAR" redefined
 #define BAR 2
 ^
hashif.h:1:0: note: this is the location of the previous definition
 #define BAR 1
 ^
hashif.h:4:2: error: #endif without #if
 #endif
  ^
# 3 "hashif.cpp" 2


extern "C" int printf(const char* fmt, ...);

int main()
{
    printf("Hello, World\n");
hashif.cpp:1:0: error: unterminated #if
 #if FOO
 ^
}