c++/qt how to have a global static variable

Question

I want to store some values / variables so that I can change and read them in any class.

What I tryed so far: I created a header file called "settings.h":

#ifndef SETTINGS_H
#define SETTINGS_H

#include <QString>

class Settings {
public:
    static QString OutputFormat;
};

#endif // SETTINGS_H

Now I included it in a class with:

#include "settings.h"

But when I try to set this variable:

Settings::OutputFormat = "mp3";

It will not compile with that error:

undefined reference to `Settings::OutputFormat'

What to do? I need something like a settings class. Any class which #include "settings.h" should be able to read the variable values and change them. The values should be global and shared between all classes which include this settings class.

See [this answer](https://stackoverflow.com/a/12574407/1413395) specifically. — πάντα ῥεῖ, Oct 13 '18 at 14:29
You wouldn’t want a static anyway, and this should be a plain struct. Expose it via an “instance” static, a-la `qApp`. — Kuba hasn't forgotten Monica, Oct 13 '18 at 16:26
@KubaOber Would be nice if you could add it as answer with some example code. (Sorry, I'm pretty c++ noob yet.) — Bleuzen, Oct 13 '18 at 16:34
If you plan to store the settings to a file (as word "settings" usually implies that in context of application programs), check out `QSettings`. — hyde, Oct 13 '18 at 19:14
@hyde no, my settings are just for program runtime and should reset when restarting the program. however thanks. — Bleuzen, Oct 13 '18 at 21:44

score 2 · Accepted Answer · answered Oct 13 '18 at 14:22

2

You need to declare a "location" for this variable in a cpp file somewhere:

QString Settings ::OutputFormat;

And this is enough. Without a "location" in a source file, the compiler will not create the variable and it will be an undefined reference.

answered Oct 13 '18 at 14:22

Matthieu Brucher

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1

I created a settings.cpp file now and did it there. Thanks. – Bleuzen Oct 13 '18 at 14:31

c++/qt how to have a global static variable

1 Answers1