How to declare a global variable in C

Question

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 realy 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

User · Answer

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.

User · Answer

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

User · Answer

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

User · Answer

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     -

User · Answer

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

[c++] How to declare a global variable in C++

Possibility 1

a.cpp

b.cpp

Possibility 2

a.cpp

b.cpp

a.cpp

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