Enum class scope resolution operator

Question

When I tried to compile test.cc by g++ test.cc --std=c++14, I got the following error.

test.cc:5:26: error: expected unqualified-id before numeric constant
Colour colour = Colour::None;

test.cc

#include "state.h"
#include <X11/X.h>

int main(){
        Colour colour = Colour::None;
}

state.h

enum class Colour { None, Black, White };

And I found that by #include <X11/X.h>, None is defined as a constant

 #define None                 0L /* universal null resource or null atom */

What bothers me is that I've already used scope resolution operator, namely Colour::None, to specify which None I'm refering, but the error still occurred.

The preprocessor doesn't care about scopes. A macro is a macro. — melpomene
– melpomene, Commented Nov 11, 2017 at 22:24

Bathsheba · Accepted Answer · 2017-11-11 22:26:24Z

6

That's life I'm afraid, and it epitomises the reasons for macros being terrible.

Once you've #included <X11/X.h>, the preprocessor will chew up your source code and the compiler will see

Colour colour = Colour::0;

which makes no sense.

One fix would be to #undef None after including the file.

edited Nov 11, 2017 at 22:26

answered Nov 11, 2017 at 22:25

Bathsheba

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2785528 · Accepted Answer · 2017-11-11 22:55:46Z

0

Did you ask a question?

Anyway, one possible direction to proceed, is to refactor your code a little bit. What I would do is

a) perform the "#include < X11/X.h >" in only 1 cpp file, perhaps call it x11x.cpp.

Then, when ever you need to use a function of the X11/X.h code,

2a) add a function declaration (i.e. perhaps 'void foo1(int)') in x11x.hpp.

2b) and implement your 'foo1(int)' to call the "X11/X.h" function in x11x.cpp.

Should have very little impact to performance.

And now the unfortunate macro will only interfere with the code of x11.cpp, so keep this code small.

edited Nov 11, 2017 at 22:55

answered Nov 11, 2017 at 22:49

2785528

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