How to share a variable between files

Question

4.00/5 (1 vote)

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So, i'm using Visual Studio 2019, C++ language.
Why does the linker give me error "unresolved external symbol" in this case?

main.cpp

C++

const int myAge = 70;

secondFile.cpp

C++

extern const int myAge;

What I have tried:

Without "const" it links fine.
Also these two variants of secondFile.cpp link successfully which doesn't make any sense to me:
secondFile.cpp

C++

const int myAge = 30;  // isn't it a duplicate symbol? Without "const" it is a a duplicate symbol!

secondFile.cpp

C++

extern const int myAge = 25; // well, it links fine but what's the point of "external" then? We can't access "myAge" from "main.cpp"!

Posted 6-Dec-20 6:31am

Avtem

Updated 14-Feb-21 18:51pm

v4

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Comments

Rick York 6-Dec-20 16:54pm

Doing this kind of thing is very bad practice. You should always avoid having constant values defined in multiple places. A better way to do this is to define the value once in a header file, without the extern.

Avtem 7-Dec-20 7:57am

But that is the point of my question. i wanted to define the variable only once and use it in all files. But i like the way you propose to achieve result i need.

2 solutions

Solution 2

The way correct, but sharing variables isnt a good idee. Better is to code some setter und getter interfaces between the files or better classes.
If you need a lot of data, than it is best to create some data providing class aka "Repository" which has a such global instance aka "SharedInstance".

Global variables are a hint to a bad design, which will lead to nasty bugs.

Posted 6-Dec-20 7:50am

KarstenK

Comments

Avtem 6-Dec-20 13:56pm

Thank you for pointing that out. Definitely something i have to work on)

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Mladen Janković · Accepted Answer · 2020-12-06T07:26:00

Solution 1

Storage class specifiers - cppreference.com[^]

Names at the top-level namespace scope (file scope in C) that are const and not extern have external linkage in C, but internal linkage in C++.

So solution to your problem would be:

// main.cpp
extern const int myAge = 70;

// secondFile.cpp
extern const int myAge;

Posted 6-Dec-20 7:26am

Mladen Janković

Comments

Avtem 6-Dec-20 13:33pm

Oh, it's interesting! i need to expand my knowledge about c++ in this direction)
Thank you very much for your answer!

Avtem 20-Jul-22 8:51am

For those, who find difficult to understand cppreference i strongly recommend this site for learning C++: After i read the article i was knowing and understanding anything from this topic:
https://www.learncpp.com/cpp-tutorial/sharing-global-constants-across-multiple-files-using-inline-variables/