Making Global Struct in C++ Program

Question

I am trying to make global structure, which will be seen from any part of the source code. I need it for my big Qt project, where some global variables needed. Here it is: 3 files (global.h, dialog.h & main.cpp). For compilation I use Visual Studio (Visual C++).

global.h

#ifndef GLOBAL_H_
#define GLOBAL_H_

typedef struct  TNumber {
    int g_nNumber;
} TNum;

TNum Num;

#endif

dialog.h

#ifndef DIALOG_H_
#define DIALOG_H_

#include <iostream>
#include "global.h"

using namespace std;

class   ClassB {
public:
    ClassB() {};

    void    showNumber() {
        Num.g_nNumber = 82;
        cout << "[ClassB][Change Number]: " << Num.g_nNumber << endl;
    }
};

#endif

and main.cpp

#include <iostream>

#include "global.h"
#include "dialog.h"

using namespace std;

class   ClassA {
public:
    ClassA() {
        cout << "Hello from class A!\n";
    };
    void    showNumber() {
        cout << "[ClassA]: " << Num.g_nNumber << endl;
    }
};

int main(int argc, char **argv) {
    ClassA  ca;
    ClassB  cb;
    ca.showNumber();
    cb.showNumber();
    ca.showNumber();
    cout << "Exit.\n";
    return 0;
}

When I`m trying to build this little application, compilation works fine, but the linker gives me back an error:

1>dialog.obj : error LNK2005: "struct TNumber Num" (?Num@@3UTNumber@@A) already defined in main.obj

Is there exists any solution?

Thanks.

Solution 1

Yes. First, Don't define num in the header file. Declare it as extern in the header and then create a file Global.cpp to store the global, or put it in main.cpp as Thomas Jones-Low's answer suggested.

Second, don't use globals.

Third, typedef is unnecessary for this purpose in C++. You can declare your struct like this:

struct  TNum {
    int g_nNumber;
};

Solution 2

In global.h

extern TNum Num;

then at the top of main.cpp

TNum Num;

Solution 3

Since you're writing in C++ use this form of declaration for a struct:

struct  TNumber {
    int g_nNumber;
};

extern TNumber Num;

The typedef is unnecessary.

Comments

[William FitzPatrick]

This answer would be much more useful if an alternative for globals were offered

[Barleyman]

@WilliamFitzPatrick Globals have their place, especially static globals which are only visible inside the specific source file. Maybe your program simply isn't million lines with hundreds of people working on it? Maybe passing that configuration object from one function to the other all over the code doesn't make sense? Maybe you just stick your configuration into one static global struct so all of your local-to-that-file functions can access the configuration which is only even written by one parsing function but read in several places.