How to access a variable from .cpp file in a .h file

Question

Consider this scenario:

File1.cpp:

double dir = M_PI/2;

hFile1.h:

void printdir () {
cout << dir;
}

Main.cpp:

#include "hFile1.h"
int main () {
printdir();
}

This obviously will not work because hFile1.h will throw an error: "use of undeclared identifier 'dir'". In this example, I want to be able to access and use the defined dir variable in hFile1.h. Is this possible?

NOTE: I have already tried using extern based on similar posts on this topic and it didn't work, even after I did exactly what they did. Code:

File1.cpp:

extern double dir = M_PI/2;

hFile1.h:

extern double dir;
void printdir () {
cout << dir;
}

Main.cpp:

#include "hFile1.h"
int main () {
printdir();
}

@HyperTextMarkupMan `#include` is just a dumb copy and paste of a file's contents, you can include any text file. Doesn't mean you should, though. — Etienne de Martel, Sep 29 '20 at 04:03
@EtiennedeMartel usually .cpp files don't contain var declaration. — asmmo, Sep 29 '20 at 04:04
@HyperTextMarkupMan `I have already tried using extern ... and it didn't work` Post *that* code, then. All you need is an `extern double dir;` declaration in either `hFile1.h` or `main.cpp`. — dxiv, Sep 29 '20 at 04:08
@HyperTextMarkupMan Because then you risk ending up with multiple definitions. — Etienne de Martel, Sep 29 '20 at 04:09
@dxiv You are correct and that is exactly what I did but I found that the value did not change in main when I changed it in File1.cpp — HyperText Markup Man, Sep 29 '20 at 04:09
@HyperTextMarkupMan Then you did something other than you say you did. That's why posting the actual code matters. — dxiv, Sep 29 '20 at 04:11
@EtiennedeMartel I did build my program but the value displayed was the old value not the changed value. — HyperText Markup Man, Sep 29 '20 at 04:11
Does this answer your question? [When to use extern in C++](https://stackoverflow.com/questions/10422034/when-to-use-extern-in-c) — Amir Kirsh, Sep 29 '20 at 04:34

score 2 · Accepted Answer · answered Sep 29 '20 at 04:13

2

You need to use keyword "extern" in hFile1.h as below. I tested, it worked.

extern double dir;

void printdir () {
    cout << dir;
}

answered Sep 29 '20 at 04:13

TuanPM

685
8
29

What I don't understand is what the extern type is *doing* in this scenario. – HyperText Markup Man Sep 29 '20 at 04:18
@HyperTextMarkupMan It's to mark `dir` has having _external linkage_. [See this question](https://stackoverflow.com/questions/1358400/what-is-external-linkage-and-internal-linkage) for more details. – Etienne de Martel Sep 29 '20 at 04:31

How to access a variable from .cpp file in a .h file

1 Answers1