Question

I have global variables that I define in the Utility namespace. This utility is included in several files and it looks like this:

#ifndef _UT_
#define _UT_
namespace UT {
  std::string PLATFORM_LINUX_NAME = "linux";
  std::string  PLATFORM_MACOSX_NAME = "macosx";
  std::string  PLATFORM_WINDOWS_NAME = "windows";
  #if defined(OS_WIN)
    int PLATFORM = OSTYPE::PLATFORM_WINDOWS;
  #elif defined(OS_LINUX)
    int PLATFORM = PLATFORM_LINUX;
  #elif defined(OS_APPLE)
    int PLATFORM = PLATFORM_MACOSX;
  #endif
};

When I include this file in for example files A.h and B.h and C.h, I’m getting a compiler warning that says:

warning LNK4006: "int UT::PLATFORM" (?PLATFORM@UT@@3HA) already defined in A.obj; second definition ignored
warning LNK4006: "class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > UT::PLATFORM_LINUX_NAME" (?PLATFORM_LINUX_NAME@UT@@3V?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@A) already defined in A.obj; second definition ignored

What is the best way that does not involve creating a class for solving this?
Or is creating a UT class the only way?

Answer 1

Define the variables in a single .cpp file and declare them in the .h file. In UT.h:

namespace UT
{
    extern const std::string PLATFORM_LINUX_NAME;
    extern const std::string PLATFORM_MACOS_NAME;
    extern const std::string PLATFORM_WINDOWS_NAME;
    extern const int PLATFORM;
}

in UT.cpp:

namespace UT
{
    const std::string PLATFORM_LINUX_NAME   = "linux";
    const std::string PLATFORM_MACOS_NAME   = "macosx";
    const std::string PLATFORM_WINDOWS_NAME = "windows";

    #if defined(OS_WIN)
    const int PLATFORM = OSTYPE::PLATFORM_WINDOWS;
    #elif defined(OS_LINUX)
    const int PLATFORM = PLATFORM_LINUX;
    #elif defined(OS_APPLE)
    const int PLATFORM = PLATFORM_MACOSX;
    #endif


}

I added const qualifier as these appear to be constant values.