C++: Switching from MSVC to G++: Global Variables

Question

I recently switched to Linux and wanted to compile my Visual Studio 2010 C++ source code, which uses only the STL, on G++.

My Linux machine currently isn't available but I can try to tell you what is going on, first:

• As I try to compile my project, all global variables I use in main and which perfectly work on MSVC result in myGlobalVar is not defined in this scope errors.

My project is built nearly the same as the example below:

// myclass.h
class myClass
{
 // ....
};
extern myClass globalInstance;

// myclass.cpp
#include "myclass.h"
// myClass functions located here
myClass globalInstance;

// main.cpp
#include "myclass.h"
int main( )
{
    // Accessing globalInstance results in an error: Not defined in this scope
}

• What am I doing wrong?
• Where are the differences between G++ and MSVC in terms of global variables?

Answer 1

Your sample code should work just fine on Linux as well as Windows. There shouldn't be any differences between GCC & MSVC with regards to visibility of global variables. I think it's more likely that what you're seeing is a symptom of another problem.

The only thing I can think off off the top of my head that might cause an issue like this would be "screwed up" header files, to use the technical term for it. A common issue in porting code from Windows to Linux is header file case sensitivity. Whereas MSVC won't care if you import MyHeader.h as #include <myheader.h> it will certainly fail on Linux. If you header isn't being included, the compiler would miss the extern declaration and might result in the error you're seeing.

Answer 2

you need to compile as follow:

g++ main.cpp myclass.cpp -o myapp

NOT as follow: g++ main.cpp -o myapp which will miss global variable declaration in myclass.cpp file.