从C ++ 98升级到C ++ 11会导致错误

Question

I am using QT Creator to make a C++ program on Ubuntu. The program I had written was compiling fine, until I decided to start using C++11 rather than C++98 (which is the default in QT Creator). I am using my own cmake file, rather than qmake, and so to do this, I included the following line in my CMakeLists.txt file :

set(CMAKE_CXX_FLAGS "-std=c++0x")

Now, part of my code has the following (which was not written by me):

#if (linux && (i386 || __x86_64__))
#   include "Linux-x86/OniPlatformLinux-x86.h"
#elif (linux && __arm__)
#   include "Linux-Arm/OniPlatformLinux-Arm.h"
#else
#   error Unsupported Platform!
#endif

After transferring to C++11, I get an error at the line error Unsupported Platform! . This is because, from what I can see, the variable linux is not defined anywhere, although the variable __x86_64__ is defined.

Therefore, I have two questions:

1) Why is the variable linux not defined, even though I am using Linux?

2) How can I tell C++11 to ignore this error?

Thanks.

Answer 1

The identifier linux is not reserved. A conforming compiler may not predefine it as a macro. For example, this program:

int main() {
    int linux = 0;
    return linux;
}

is perfectly valid, and a conforming compiler must accept it. Predefining linux causes the declaration to be a syntax error.

Some older compilers (including the compiler you were using, with the options you were giving it) predefine certain symbols to provide information about the target platform -- including linux to indicate a Linux system. This convention goes back to early C compilers, written before there was a distinction between reserved and unreserved identifiers.

The identifier __linux__ , since it starts with two underscores, is reserved for use by the implementation, so compilers are allowed to predefine it -- and compilers for Linux systems typically do predefine it as a macro expanding to 1 .

Confirm that your compiler predefines __linux__ , and then change your code so it tests __linux__ rather than linux . You should also find out what reserved symbol is used instead of i386 (likely __i386__ ).

Related: Why does the C preprocessor interpret the word "linux" as the constant "1"?

Answer 2

Change your standard-selection flag to -std=gnu++0x instead of c++0x . The gnu flavors provide some non-standard extensions, apparently including predefining the macro linux . Alternatively, check for __linux__ instead.