Any way I can define and declare an extern object in header?

Question

I'm writing a library which is totally base on templates, so I don't have any cpp files. Now I want to declare an global variable, then I realize I have nowhere to go.

If I simply declared it in header, I will got a "multiple definition" error, if I use extern, I have to create a cpp file to really declare it.

So is there any way I can declare a global variable in header?

P.S. since a static member in a template class can (only) be declared in header, how it works?

@M.M, what if I'm using C++ 11? Any way to do this? – Dec 05 '19 at 04:07 — , Dec 05 '19 at 04:07

score 1 · Answer 1 · answered Dec 05 '19 at 04:10

1

You can use macro to for single declaration,

#ifndef __usermacro
#define __usermacro
//Declare global variable
#else
//Declare extern 
#endif

answered Dec 05 '19 at 04:10

TruthSeeker

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`#ifnedf` do not share between translation units, this does not work – Dec 05 '19 at 04:23
Yes, that's true. In single translation unit you can avoid multiple instances of the global variable – TruthSeeker Dec 05 '19 at 04:41

Denis Sheremet · Accepted Answer · 2019-12-05T04:57:47.577

As @M.M mentioned, you can use inline declaration if you're on C++17 or above.

Hovever, if that's not the case, you can declare inline function which returns a reference to static variable, just like that:

inline int& getData() {
    static int data;
    return data;
}

Then, in your .cpp file (as well as in any function body inside your headers) you can simply call it like int& data = getData().

As a side note, if you want to ensure that global object is created only once and is not copied by accident, it could suit you better to use a signleton instead. Global variables are more of a c-style and not really considered a good practice in c++.

Any way I can define and declare an extern object in header?

2 Answers2