static functions

Hi,

I have a function which I need in different source files again and again. So I thought implementing it in an own class (Defaults) as static function. I have

Code:

---

//Defaults.h
#ifndef DEFAULTS_H
#define DEFAULTS_H

class Defaults
{
  public:
  static const double c;
  static void read();
};

#endif

---

and

Code:

---

//defaults.cpp
#include "defaults.h"
#include <iostream>

using namespace std;

static void Defaults::read() //this is line no. 6
{
  cout << "static read";
}

---

And then somewhere in my code I want to do:

Code:

---

//some other class.cpp
#include "mainwindow.h"
#include "defaults.h"
#include <iostream>

using namespace std;

MainWindow::MainWindow()
{
cout << "construtctor mainwindow";
cout << Defaults::c; // c was set before this works

Defaults::read(); // this doesn't work
}

---

When trying to compile I get

Quote:

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defaults.cpp:6: error: cannot declare member function ‘static void Defaults::read()’ to have static linkage

---

So what is wrong here?

Apart from the bug itself here, is that the right way to achieve what I want to achieve?

Thanks, J.

Putting helper functions in a separate class and make them static is what I do sometimes as well, so I'd say, it's not a bad idea.

As to your problem: omit the 'static' in the .cpp file. It is only needed in the header file .

Quote:

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Originally Posted by joebar

So what is wrong here?

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You don't need to specify the static keyword before the function definition, only before the declaration in the header file.

Quote:

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Apart from the bug itself here, is that the right way to achieve what I want to achieve?

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I don't know what you want to achieve. However, what you have here is nothing but a glorified global variable. There is little use for a class with only static members (unless the class is designed to be used as a template argument, e.g. in policy based design). Instead you can use a namespace to prevent polluting the global namespace. But then still, you are using a global variable, which is often a sign of bad design.

Thanks, the bug itself is resovled.

I want to use this class to define glober helper functions I need again and again in different classes, files, etc. So if there is a better way of doing so I am open to it.

Typically the way to design reusable helper functions is to carefully distill the interface to be as general as possible, and then stick it in an appropriate header file.

Quote:

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Originally Posted by D_Drmmr

But then still, you are using a global variable, which is often a sign of bad design.

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Global const variable. AFAIK, there is nothing wrong with those. For example: std::string::npos, std::ios::in, etc...

Quote:

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Originally Posted by monarch_dodra

Global const variable. AFAIK, there is nothing wrong with those. For example: std::string::npos, std::ios::in, etc...

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You're right. I didn't notice the const and assumed the read() function would load the value from file or something.