Why is this macro generating a syntax error? - c++

Question

So this is my code:
// Defines a tuple
#define __WINDOW__RESOLUTION__ 500, 200
// Seperate the tuple
#define __WINDOW__X__1(Width, Height) (Width)
#define __WINDOW__Y__1(Width, Height) (Height)
// Add another sort of indirection because my tuple is a macro
#define __WINDOW__X__(Macro) __WINDOW__X__1(Macro)
#define __WINDOW__Y__(Macro) __WINDOW__Y__1(Macro)
// These should be the final values 500 and 200
#define __WINDOW__RESOLUTION__X__ (__WINDOW__X__(__WINDOW__RESOLUTION__))
#define __WINDOW__RESOLUTION__Y__ (__WINDOW__Y__(__WINDOW__RESOLUTION__))
When i use the first macro where the final number should be something seems to go wrong:
std::cout << __WINDOW__RESOLUTION__X__ << std::endl; // Outputs 200 instead of 500
above line outputs the number 200, so the Y value instead of the X value
std::cout << __WINDOW__RESOLUTION__Y__ << std::endl; // ERR with macro underlined
this line won't even compile [ C2059, syntax error: ")" ]
Thank you for helping me Alex

Answer 1

For the record - I understand that your solution is "do not do this", but I still wanted to provide an answer for the original question.
In fact, I actually don't see anything wrong with your code. It actually works fine if you compile it with a basic test example like this one:
// Defines a tuple
#define __WINDOW__RESOLUTION__ 500, 200
// Seperate the tuple
#define __WINDOW__X__1(Width, Height) (Width)
#define __WINDOW__Y__1(Width, Height) (Height)
// Add another sort of indirection because my tuple is a macro
#define __WINDOW__X__(Macro) __WINDOW__X__1(Macro)
#define __WINDOW__Y__(Macro) __WINDOW__Y__1(Macro)
// These should be the final values 500 and 200
#define __WINDOW__RESOLUTION__X__ (__WINDOW__X__(__WINDOW__RESOLUTION__))
#define __WINDOW__RESOLUTION__Y__ (__WINDOW__Y__(__WINDOW__RESOLUTION__))
#include <iostream>
using namespace std;
int main() {
// your code goes here
std::cout << __WINDOW__RESOLUTION__X__ << std::endl;
std::cout << __WINDOW__RESOLUTION__Y__ << std::endl;
return 0;
}
It looks like your error may be related to the brackets in your last macro, though, so just removing them could possibly solve that, but again - this wasn't required for me (using gcc 5.4 under ubuntu):
// These should be the final values 500 and 200
#define __WINDOW__RESOLUTION__X__ __WINDOW__X__(__WINDOW__RESOLUTION__)
#define __WINDOW__RESOLUTION__Y__ __WINDOW__Y__(__WINDOW__RESOLUTION__)
It was previously also pointed out that double underscore __ followed by an upper-case letter is reserved, but it certainly didn't stop gcc from compiling this - maybe it would generate an error with stricter compile flags, though!

Answer 2

It seems to work fine when compiled with gcc4.9, but the problem might lie in that the compiler threads __WINDOW__RESOLUTION__ as a single parameter. For example, if you replace
#define __WINDOW__RESOLUTION__X__ (__WINDOW__X__(__WINDOW__RESOLUTION__))
with
#define __WINDOW__RESOLUTION__X__ (__WINDOW__X__(500, 200))
It will throw an error because __WINDOW__X__ expects only 1 parameter.
You can solve this by using ... and __VA_ARGS__ in your macro, which will forward any parameter that it receives:
#define __WINDOW__X__(...) __WINDOW__X__1(__VA_ARGS__)
#define __WINDOW__Y__(...) __WINDOW__Y__1(__VA_ARGS__)