How to declare a global variable in C++

I know one should not use global variables but I have a need for them. I have read that any variable declared outside a function is a global variable. I have done so, but in another *.cpp file, that variable could not be found. So it was not really global. Isn't it so that one has to create a header file GlobalVariabels.h and include that file to any other *cpp file that uses it?

Solution 1:

I have read that any variable declared outside a function is a global variable. I have done so, but in another *.cpp File that variable could not be found. So it was not realy global.

According to the concept of scope, your variable is global. However, what you've read/understood is overly-simplified.

Possibility 1

Perhaps you forgot to declare the variable in the other translation unit (TU). Here's an example:

a.cpp

int x = 5; // declaration and definition of my global variable

b.cpp

// I want to use `x` here, too.
// But I need b.cpp to know that it exists, first:
extern int x; // declaration (not definition)

void foo() {
   cout << x;  // OK
}

Typically you'd place extern int x; in a header file that gets included into b.cpp, and also into any other TU that ends up needing to use x.

Possibility 2

Additionally, it's possible that the variable has internal linkage, meaning that it's not exposed across translation units. This will be the case by default if the variable is marked const ([C++11: 3.5/3]):

a.cpp

const int x = 5; // file-`static` by default, because `const`

b.cpp

extern const int x;    // says there's a `x` that we can use somewhere...

void foo() {
   cout << x;    // ... but actually there isn't. So, linker error.
}

You could fix this by applying extern to the definition, too:

a.cpp

extern const int x = 5;

This whole malarky is roughly equivalent to the mess you go through making functions visible/usable across TU boundaries, but with some differences in how you go about it.

Solution 2:

You declare the variable as extern in a common header:

//globals.h
extern int x;

And define it in an implementation file.

//globals.cpp
int x = 1337;

You can then include the header everywhere you need access to it.

I suggest you also wrap the variable inside a namespace.

Solution 3:

In addition to other answers here, if the value is an integral constant, a public enum in a class or struct will work. A variable - constant or otherwise - at the root of a namespace is another option, or a static public member of a class or struct is a third option.

MyClass::eSomeConst (enum)
MyNamespace::nSomeValue 
MyStruct::nSomeValue (static)