How to declare a global variable in C++

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.

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.

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) 

Declare extern int x; in file.h. And define int x; only in one cpp file.cpp.

Not sure if this is correct in any sense but this seems to work for me.

someHeader.h inline int someVar; 

I don't have linking/multiple definition issues and it "just works"... ;- )

It's quite handy for "quick" tests... Try to avoid global vars tho, because every says so... ;- )