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

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

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

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

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  lt  lt  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  lt  lt  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

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++

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