Defining compiler directive in different file C++

Question

Consider following code snippet,

    // header file
    #define TEXT_ENUM                //////////// HERE
    #ifdef TEXT_ENUM
        #define TEXT_HANDLING_MACRO  \
             static const char * TEXT[]; \
             static const char * getText( int _enum ) { \
                 return TEXT[_enum]; \
             }
    #else
        #define TEXT_HANDLING_MACRO
    #endif

    struct Foo {
        TEXT_HANDLING_MACRO
    };

   // cpp file
   #include "foo.h"
   #ifdef TEXT_ENUM
       const char * Foo::TEXT[] = { 
           "ONE",
           "TWO",
           "THREE",
           0
       };
   #endif

   // other_file.cpp
   #include <iostream>
   #include "foo.h"
   void bar() {
       std::cout << Foo::TEXT[0] <<std::endl;
   }

It works perfectly fine, but in following code it doesn't,

    // header file
    #ifdef TEXT_ENUM
        #define TEXT_HANDLING_MACRO  \
             static const char * TEXT[]; \
             static const char * getText( int _enum ) { \
                 return TEXT[_enum]; \
             }
    #else
        #define TEXT_HANDLING_MACRO
    #endif

    struct Foo {
        TEXT_HANDLING_MACRO
    };

   // cpp file
   #include "foo.h"
   #ifdef TEXT_ENUM
       const char * Foo::TEXT[] = { 
           "ONE",
           "TWO",
           "THREE",
           0
       };
   #endif

   // other_file.cpp
   #include <iostream>
   #define TEXT_ENUM                //////////// HERE
   #include "foo.h"
   void bar() {
       std::cout << Foo::TEXT[0] <<std::endl;
   }

The difference is the position of #include TEXT_ENUM, I get the error of undefined reference to Foo::TEXT. I have defined TEXT_ENUM before including the file.

How can I resolve it?

What's the difference? Can't spot it? Are you sure you're linking `.o` files in the right order? — πάντα ῥεῖ, Oct 23 '16 at 18:02
Your second example breaks the [one definition rule](https://en.wikipedia.org/wiki/One_Definition_Rule). You will have two different definitions of the `Foo` structure in different translation units. — Some programmer dude, Oct 23 '16 at 18:05
Also, macros are defined per translation unit. Macros defined in one translation unit isn't available in another. — Some programmer dude, Oct 23 '16 at 18:06
The position of `#define TEXT_ENUM` is different and is marked by `////////////// HERE`. — Oli, Oct 23 '16 at 18:12
I added '#pargma warning ", Qt moc file is not expanding macro call which is strange. — Oli, Oct 24 '16 at 11:37

Defining compiler directive in different file C++

0 Answers0