user17310: I am trying to write a library for encoders.
main.ino:
#include <./Encoders.h>

Encoders encoders();

void setup(){
    Serial.begin(9600);
}

void loop(){               
    Serial.print("");
}

Encoders.h:
#ifndef Encoders_h
#define Encoders_h

#include "Arduino.h"

class Encoders{
    public:
        Encoders();
        void EncodersSetup();
        long RightCount;
        long LeftCount;
        void R_A_RISE();
        void R_A_FALL();
        void R_B_RISE();
        void R_B_FALL();
        void L_A_RISE();
        void L_A_FALL();
        void L_B_RISE();
        void L_B_FALL();
    private:
        short R_A_SIG, R_B_SIG, L_A_SIG, L_B_SIG;
};

#endif

Encoders.cpp
#include "Arduino.h"
#include "Encoders.h"

long RightCount;
long LeftCount;
long R_A_SIG, R_B_SIG, L_A_SIG, L_B_SIG;

Encoders::Encoders(){
    attachInterrupt(0, R_A_RISE, RISING);
    attachInterrupt(1, R_B_RISE, RISING);
    attachInterrupt(2, L_A_RISE, RISING);
    attachInterrupt(3, L_B_RISE, RISING);
    R_A_RISE();
}

void Encoders::R_A_RISE(){
    detachInterrupt(0);
    R_A_SIG = 1;
    if(R_B_SIG == 0){
        RightCount++;
    }
    if(R_B_SIG == 1){
        RightCount--;
    }
    attachInterrupt(0, R_A_FALL, FALLING);
}
void Encoders::R_A_FALL(){
    detachInterrupt(0);
    R_A_SIG = 0;
    if(R_B_SIG == 1){
        RightCount++;
    } 
    if(R_B_SIG == 0){
        RightCount--;
    }
    attachInterrupt(0, R_A_RISE, RISING);
}
void Encoders::R_B_RISE(){
    detachInterrupt(1);
    R_B_SIG = 1;
    if(R_A_SIG == 1){
        RightCount++;
    }
    if(R_A_SIG == 0){
        RightCount--;
    }
    attachInterrupt(1, R_B_FALL, FALLING);
}
void Encoders::R_B_FALL(){
    detachInterrupt(1);
    R_B_SIG = 0;
    if(R_A_SIG == 0){
        RightCount++;
    }
    if(R_A_SIG == 1){
        RightCount--;
    }   
    attachInterrupt(1, R_B_RISE, RISING);
}
void Encoders::L_A_RISE(){
    detachInterrupt(2);
    L_A_SIG = 1;
    if(L_B_SIG == 0){
        LeftCount++;
    }
    if(L_B_SIG == 1){
        LeftCount--;
    }
    attachInterrupt(2, L_A_FALL, FALLING);
}
void Encoders::L_A_FALL(){
    detachInterrupt(2);
    L_A_SIG = 0;
    if(L_B_SIG == 1){
        LeftCount++;
        } 
    if(L_B_SIG == 0){
        LeftCount--;
        }
    attachInterrupt(2, L_A_RISE, RISING);   
}
void Encoders::L_B_RISE(){
    detachInterrupt(3);
    L_B_SIG = 1;
    if(L_A_SIG == 1){
        LeftCount++;
    }
    if(L_A_SIG == 0){
        LeftCount--;
    }
    attachInterrupt(3, L_B_FALL, FALLING);
}
void Encoders::L_B_FALL(){
    detachInterrupt(3);
    L_B_SIG = 0;
    if(L_A_SIG == 0){
        LeftCount++;
    }
    if(L_A_SIG == 1){
        LeftCount--;
    }   
    attachInterrupt(3, L_B_RISE, RISING);
}

When I try to compile this, I get errors like this:
C:/Users/marcel/Documents/dev/Arduino/encodeurs/Encoders.cpp:10:38: error: cannot convert 'Encoders::R_B_RISE' from type 'void (Encoders::)()' to type 'void (*)()'

I tried answers on other threads. Most said to make the functions passed to attachInterrupt() static. That threw different errors:
C:/Users/marcel/Documents/dev/Arduino/encodeurs/Encoders.cpp:16:33: error: cannot declare member function 'void Encoders::R_A_RISE()' to have static linkage [-fpermissive]

I also tried this:
attachInterrupt(0, &Encoders::R_A_RISE, this, RISING);

with no success.
The Arduino in question is a Mega but I doubt that matters.
Edit: Per Nick Gammon's answer, I now have this.
MotorEncoderMaster.ino
#include </home/marcel/dev/Arduino/encoders/MotorEncoderMaster/Encoders.h>
Encoders * Encoders::instances [2] = { NULL, NULL };


// instances of our class
Encoders right;
Encoders left;

void setup (){
    right.begin (2);   // pin D2
    left.begin (3);   // pin D3
}  // end of setup

void loop (){
    // whatever
}  // end of loop

Encoders.h:
class Encoders{
    volatile bool switchChanged;

    static Encoders * instances [2];

    static void switchPressedExt0(){
        if(Encoders::instances [0] != NULL){
            Encoders::instances [0]->switchPressed();
        }
    }

    static void switchPressedExt1 (){
        if (Encoders::instances [1] != NULL){
            Encoders::instances [1]->switchPressed();
        }
    }


    public:
        void begin (const int whichPin){
            pinMode (whichPin, INPUT_PULLUP);
            switch (whichPin){
                case 2:
                    attachInterrupt(0, switchPressedExt0, CHANGE);
                    instances [0] = this;
                    break;

                case 3:
                    attachInterrupt(1, switchPressedExt1, CHANGE);
                    instances [1] = this;
                    break;

            }
        }

        void switchPressed(){
            switchChanged = true;
        }

};

When I try to compile this, I get :
In file included from MotorEncoderMaster.ino:1:0:
/home/marcel/dev/Arduino/encoders/Encoders.h: In static member function ‘static void Encoders::switchPressedExt0()’:
/home/marcel/dev/Arduino/encoders/Encoders.h:7:34: error: ‘NULL’ was not declared in this scope
    if(Encoders::instances [0] != NULL){
                                  ^
/home/marcel/dev/Arduino/encoders/Encoders.h: In static member function ‘static void Encoders::switchPressedExt1()’:
/home/marcel/dev/Arduino/encoders/Encoders.h:13:35: error: ‘NULL’ was not declared in this scope
    if (Encoders::instances [1] != NULL){
                                   ^
/home/marcel/dev/Arduino/encoders/Encoders.h: In member function ‘void Encoders::begin(int)’:
/home/marcel/dev/Arduino/encoders/Encoders.h:21:24: error: ‘INPUT_PULLUP’ was not declared in this scope
     pinMode (whichPin, INPUT_PULLUP);
                        ^
/home/marcel/dev/Arduino/encoders/Encoders.h:21:36: error: ‘pinMode’ was not declared in this scope
     pinMode (whichPin, INPUT_PULLUP);
                                    ^
/home/marcel/dev/Arduino/encoders/Encoders.h:24:45: error: ‘CHANGE’ was not declared in this scope
       attachInterrupt(0, switchPressedExt0, CHANGE);
                                             ^
/home/marcel/dev/Arduino/encoders/Encoders.h:24:51: error: ‘attachInterrupt’ was not declared in this scope
       attachInterrupt(0, switchPressedExt0, CHANGE);
                                                   ^


user10794: Interrupt Service Routine (ISR) outside a class
Let's consider a simple use of interrupts:
volatile bool switchChanged;

void switchPressed ()
  {
  switchChanged = true;
  }  // end of switchPressed

void setup ()
  {
  pinMode (2, INPUT_PULLUP);
  attachInterrupt (0, switchPressed, CHANGE);
  }  // end of setup

void loop ()
  {
  // whatever    
  }  // end of loop

That compiles fine.

ISR inside a class as a class function (method)
Now let's imagine we want to put the interrupt handling into a class for our convenience.
class myClass
  {
  volatile bool switchChanged;

  public:

  void begin ()
    {
    pinMode (2, INPUT_PULLUP);
    attachInterrupt (0, switchPressed, CHANGE);   // <--- line 10
    }  // end of myClass::begin

  void switchPressed ()
    {
    switchChanged = true;
    }  // end of myClass::switchPressed

  };  // end of class myClass

myClass foo;  // make an instance of myClass

void setup ()
  {
  foo.begin ();
  }  // end of setup

void loop ()
  {
  // whatever    
  }  // end of loop

That does not compile:
ISR_in_class_test.ino: In member function ‘void myClass::begin()’:
ISR_in_class_test:10: error: argument of type ‘void (myClass::)()’ does not match ‘void (*)()’

What is going on here?
ISRs have to be static functions, taking no arguments. However (non-static) class functions have an implied this-> pointer which points to the particular instance of the class.
For example, if we have two instances:
myClass foo;  
myClass bar; 

If we call foo.begin() then this-> points to "foo", and if we call bar.begin then this-> points to "bar".
However an ISR, when fired by the processor, cannot know whether this-> is "foo" or "bar" or something else. Thus the compiler cannot compile that line.

ISR inside a class as a static class function
We can try to work around this by making the class function static. Doing that means that the function is not tied to any particular instance, and thus the attachInterrupt line will compile.
class myClass
  {
  volatile bool switchChanged;   // <--- line 3

  public:

  void begin ()
    {
    pinMode (2, INPUT_PULLUP);
    attachInterrupt (0, switchPressed, CHANGE);
    }  // end of myClass::begin

  static void switchPressed ()
    {
    switchChanged = true;   // <--- line 15
    }  // end of myClass::switchPressed

  };  // end of class myClass

myClass foo;  // make an instance of myClass

void setup ()
  {
  foo.begin ();
  }  // end of setup

void loop ()
  {
  // whatever    
  }  // end of loop

However now we have a different problem:
ISR_in_class_test.ino: In static member function ‘static void myClass::switchPressed()’:
ISR_in_class_test:3: error: invalid use of member ‘myClass::switchChanged’ in static member function
ISR_in_class_test:15: error: from this location

A non-static class variable cannot be called from a static class function. Why? Because the compiler doesn't know which variable you want. Is it foo.switchChanged or bar.switchChanged?

ISR inside a class as a static class function with static variables
To make the static function work, it can only access static variables. So we can make switchChanged static. Plus we need to define an instance of this static variable.
class myClass
  {
  static volatile bool switchChanged;  // declare

  public:

  void begin ()
    {
    pinMode (2, INPUT_PULLUP);
    attachInterrupt (0, switchPressed, CHANGE);
    }  // end of myClass::begin

  static void switchPressed ()
    {
    switchChanged = true;
    }  // end of myClass::switchPressed

  };  // end of class myClass

volatile bool myClass::switchChanged;  // define

myClass foo;  // make an instance of myClass

void setup ()
  {
  foo.begin ();
  }  // end of setup

void loop ()
  {
  // whatever    
  }  // end of loop

Now the class compiles. However we have thrown away most of the advantages of having a class in the first place, as we are forced to use a static function, and that static function can only access static variables.

Glue routines
To work around this problem we can write short "glue" routines. These are functions that interface between an ISR and an instance of a class.
class myClass
  {
  volatile bool switchChanged;

  static myClass * instances [2];

  static void switchPressedExt0 ()
    {
    if (myClass::instances [0] != NULL)
      myClass::instances [0]->switchPressed ();
    }  // end of myClass::switchPressedExt0

  static void switchPressedExt1 ()
    {
    if (myClass::instances [1] != NULL)
      myClass::instances [1]->switchPressed ();
    }  // end of myClass::switchPressedExt1


  public:

  void begin (const byte whichPin)
    {
    pinMode (whichPin, INPUT_PULLUP);
    switch (whichPin)
      {
      case 2: 
        attachInterrupt (0, switchPressedExt0, CHANGE);
        instances [0] = this;
        break;

      case 3: 
        attachInterrupt (1, switchPressedExt1, CHANGE);
        instances [1] = this;
        break;

      } // end of switch
    }  // end of myClass::begin

  void switchPressed ()
    {
    switchChanged = true; 
    }

  };  // end of class myClass

myClass * myClass::instances [2] = { NULL, NULL };

// instances of our class  
myClass foo; 
myClass bar;

void setup ()
  {
  foo.begin (2);   // pin D2
  bar.begin (3);   // pin D3
  }  // end of setup

void loop ()
  {
  // whatever    
  }  // end of loop

This is a bit fiddly, however what it is doing is remembering in an array which instance of the class is associated with which interrupt. The "glue" routines switchPressedExt0 and switchPressedExt1 call the appropriate instance of the switchPressed function by using the remembered class pointer.
Now the non-static function switchPressed can access non-static class variables.

Adapted from my web page Calling an ISR from a class

17310: <p>I am trying to write a library for encoders.</p>
<p>main.ino:</p>
<pre><code>#include &lt;./Encoders.h&gt;

Encoders encoders();

void setup(){
    Serial.begin(9600);
}

void loop(){               
    Serial.print(&quot;&quot;);
}
</code></pre>
<p>Encoders.h:</p>
<pre><code>#ifndef Encoders_h
#define Encoders_h

#include &quot;Arduino.h&quot;

class Encoders{
    public:
        Encoders();
        void EncodersSetup();
        long RightCount;
        long LeftCount;
        void R_A_RISE();
        void R_A_FALL();
        void R_B_RISE();
        void R_B_FALL();
        void L_A_RISE();
        void L_A_FALL();
        void L_B_RISE();
        void L_B_FALL();
    private:
        short R_A_SIG, R_B_SIG, L_A_SIG, L_B_SIG;
};

#endif
</code></pre>
<p>Encoders.cpp</p>
<pre><code>#include &quot;Arduino.h&quot;
#include &quot;Encoders.h&quot;

long RightCount;
long LeftCount;
long R_A_SIG, R_B_SIG, L_A_SIG, L_B_SIG;

Encoders::Encoders(){
    attachInterrupt(0, R_A_RISE, RISING);
    attachInterrupt(1, R_B_RISE, RISING);
    attachInterrupt(2, L_A_RISE, RISING);
    attachInterrupt(3, L_B_RISE, RISING);
    R_A_RISE();
}

void Encoders::R_A_RISE(){
    detachInterrupt(0);
    R_A_SIG = 1;
    if(R_B_SIG == 0){
        RightCount++;
    }
    if(R_B_SIG == 1){
        RightCount--;
    }
    attachInterrupt(0, R_A_FALL, FALLING);
}
void Encoders::R_A_FALL(){
    detachInterrupt(0);
    R_A_SIG = 0;
    if(R_B_SIG == 1){
        RightCount++;
    } 
    if(R_B_SIG == 0){
        RightCount--;
    }
    attachInterrupt(0, R_A_RISE, RISING);
}
void Encoders::R_B_RISE(){
    detachInterrupt(1);
    R_B_SIG = 1;
    if(R_A_SIG == 1){
        RightCount++;
    }
    if(R_A_SIG == 0){
        RightCount--;
    }
    attachInterrupt(1, R_B_FALL, FALLING);
}
void Encoders::R_B_FALL(){
    detachInterrupt(1);
    R_B_SIG = 0;
    if(R_A_SIG == 0){
        RightCount++;
    }
    if(R_A_SIG == 1){
        RightCount--;
    }   
    attachInterrupt(1, R_B_RISE, RISING);
}
void Encoders::L_A_RISE(){
    detachInterrupt(2);
    L_A_SIG = 1;
    if(L_B_SIG == 0){
        LeftCount++;
    }
    if(L_B_SIG == 1){
        LeftCount--;
    }
    attachInterrupt(2, L_A_FALL, FALLING);
}
void Encoders::L_A_FALL(){
    detachInterrupt(2);
    L_A_SIG = 0;
    if(L_B_SIG == 1){
        LeftCount++;
        } 
    if(L_B_SIG == 0){
        LeftCount--;
        }
    attachInterrupt(2, L_A_RISE, RISING);   
}
void Encoders::L_B_RISE(){
    detachInterrupt(3);
    L_B_SIG = 1;
    if(L_A_SIG == 1){
        LeftCount++;
    }
    if(L_A_SIG == 0){
        LeftCount--;
    }
    attachInterrupt(3, L_B_FALL, FALLING);
}
void Encoders::L_B_FALL(){
    detachInterrupt(3);
    L_B_SIG = 0;
    if(L_A_SIG == 0){
        LeftCount++;
    }
    if(L_A_SIG == 1){
        LeftCount--;
    }   
    attachInterrupt(3, L_B_RISE, RISING);
}
</code></pre>
<p>When I try to compile this, I get errors like this:</p>
<pre><code>C:/Users/marcel/Documents/dev/Arduino/encodeurs/Encoders.cpp:10:38: error: cannot convert 'Encoders::R_B_RISE' from type 'void (Encoders::)()' to type 'void (*)()'
</code></pre>
<p>I tried answers on other threads. Most said to make the functions passed to <code>attachInterrupt()</code> <code>static</code>. That threw different errors:</p>
<pre><code>C:/Users/marcel/Documents/dev/Arduino/encodeurs/Encoders.cpp:16:33: error: cannot declare member function 'void Encoders::R_A_RISE()' to have static linkage [-fpermissive]
</code></pre>
<p>I also tried this:</p>
<pre><code>attachInterrupt(0, &amp;Encoders::R_A_RISE, this, RISING);
</code></pre>
<p>with no success.</p>
<p>The Arduino in question is a Mega but I doubt that matters.</p>
<h1>Edit: Per Nick Gammon's answer, I now have this.</h1>
<p>MotorEncoderMaster.ino</p>
<pre><code>#include &lt;/home/marcel/dev/Arduino/encoders/MotorEncoderMaster/Encoders.h&gt;
Encoders * Encoders::instances [2] = { NULL, NULL };


// instances of our class
Encoders right;
Encoders left;

void setup (){
    right.begin (2);   // pin D2
    left.begin (3);   // pin D3
}  // end of setup

void loop (){
    // whatever
}  // end of loop
</code></pre>
<p>Encoders.h:</p>
<pre><code>class Encoders{
    volatile bool switchChanged;

    static Encoders * instances [2];

    static void switchPressedExt0(){
        if(Encoders::instances [0] != NULL){
            Encoders::instances [0]-&gt;switchPressed();
        }
    }

    static void switchPressedExt1 (){
        if (Encoders::instances [1] != NULL){
            Encoders::instances [1]-&gt;switchPressed();
        }
    }


    public:
        void begin (const int whichPin){
            pinMode (whichPin, INPUT_PULLUP);
            switch (whichPin){
                case 2:
                    attachInterrupt(0, switchPressedExt0, CHANGE);
                    instances [0] = this;
                    break;

                case 3:
                    attachInterrupt(1, switchPressedExt1, CHANGE);
                    instances [1] = this;
                    break;

            }
        }

        void switchPressed(){
            switchChanged = true;
        }

};
</code></pre>
<p>When I try to compile this, I get :</p>
<pre><code>In file included from MotorEncoderMaster.ino:1:0:
/home/marcel/dev/Arduino/encoders/Encoders.h: In static member function ‘static void Encoders::switchPressedExt0()’:
/home/marcel/dev/Arduino/encoders/Encoders.h:7:34: error: ‘NULL’ was not declared in this scope
    if(Encoders::instances [0] != NULL){
                                  ^
/home/marcel/dev/Arduino/encoders/Encoders.h: In static member function ‘static void Encoders::switchPressedExt1()’:
/home/marcel/dev/Arduino/encoders/Encoders.h:13:35: error: ‘NULL’ was not declared in this scope
    if (Encoders::instances [1] != NULL){
                                   ^
/home/marcel/dev/Arduino/encoders/Encoders.h: In member function ‘void Encoders::begin(int)’:
/home/marcel/dev/Arduino/encoders/Encoders.h:21:24: error: ‘INPUT_PULLUP’ was not declared in this scope
     pinMode (whichPin, INPUT_PULLUP);
                        ^
/home/marcel/dev/Arduino/encoders/Encoders.h:21:36: error: ‘pinMode’ was not declared in this scope
     pinMode (whichPin, INPUT_PULLUP);
                                    ^
/home/marcel/dev/Arduino/encoders/Encoders.h:24:45: error: ‘CHANGE’ was not declared in this scope
       attachInterrupt(0, switchPressedExt0, CHANGE);
                                             ^
/home/marcel/dev/Arduino/encoders/Encoders.h:24:51: error: ‘attachInterrupt’ was not declared in this scope
       attachInterrupt(0, switchPressedExt0, CHANGE);
                                                   ^
</code></pre>
 10794: 

<h1>Interrupt Service Routine (ISR) outside a class</h1>

<p>Let's consider a simple use of interrupts:</p>

<pre class="lang-C++ prettyprint-override"><code>volatile bool switchChanged;

void switchPressed ()
  {
  switchChanged = true;
  }  // end of switchPressed

void setup ()
  {
  pinMode (2, INPUT_PULLUP);
  attachInterrupt (0, switchPressed, CHANGE);
  }  // end of setup

void loop ()
  {
  // whatever    
  }  // end of loop
</code></pre>

<p>That compiles fine.</p>

<hr>

<h1>ISR inside a class as a class function (method)</h1>

<p>Now let's imagine we want to put the interrupt handling into a class for our convenience.</p>

<pre class="lang-C++ prettyprint-override"><code>class myClass
  {
  volatile bool switchChanged;

  public:

  void begin ()
    {
    pinMode (2, INPUT_PULLUP);
    attachInterrupt (0, switchPressed, CHANGE);   // &lt;--- line 10
    }  // end of myClass::begin

  void switchPressed ()
    {
    switchChanged = true;
    }  // end of myClass::switchPressed

  };  // end of class myClass

myClass foo;  // make an instance of myClass

void setup ()
  {
  foo.begin ();
  }  // end of setup

void loop ()
  {
  // whatever    
  }  // end of loop
</code></pre>

<p>That does <strong>not</strong> compile:</p>

<pre class="lang-C++ prettyprint-override"><code>ISR_in_class_test.ino: In member function ‘void myClass::begin()’:
ISR_in_class_test:10: error: argument of type ‘void (myClass::)()’ does not match ‘void (*)()’
</code></pre>

<p>What is going on here?</p>

<p>ISRs have to be static functions, taking no arguments. However (non-static) class functions have an implied <code>this-&gt;</code> pointer which points to the particular <em>instance</em> of the class.</p>

<p>For example, if we have two instances:</p>

<pre class="lang-C++ prettyprint-override"><code>myClass foo;  
myClass bar; 
</code></pre>

<p>If we call foo.begin() then <code>this-&gt;</code> points to "foo", and if we call bar.begin then <code>this-&gt;</code> points to "bar".</p>

<p>However an ISR, when fired by the processor, cannot know whether <code>this-&gt;</code> is "foo" or "bar" or something else. Thus the compiler cannot compile that line.</p>

<hr>

<h1>ISR inside a class as a static class function</h1>

<p>We can try to work around this by making the class function <strong>static</strong>. Doing that means that the function is not tied to any particular instance, and thus the attachInterrupt line will compile.</p>

<pre class="lang-C++ prettyprint-override"><code>class myClass
  {
  volatile bool switchChanged;   // &lt;--- line 3

  public:

  void begin ()
    {
    pinMode (2, INPUT_PULLUP);
    attachInterrupt (0, switchPressed, CHANGE);
    }  // end of myClass::begin

  static void switchPressed ()
    {
    switchChanged = true;   // &lt;--- line 15
    }  // end of myClass::switchPressed

  };  // end of class myClass

myClass foo;  // make an instance of myClass

void setup ()
  {
  foo.begin ();
  }  // end of setup

void loop ()
  {
  // whatever    
  }  // end of loop
</code></pre>

<p>However now we have a different problem:</p>

<pre class="lang-C++ prettyprint-override"><code>ISR_in_class_test.ino: In static member function ‘static void myClass::switchPressed()’:
ISR_in_class_test:3: error: invalid use of member ‘myClass::switchChanged’ in static member function
ISR_in_class_test:15: error: from this location
</code></pre>

<p>A non-static class variable cannot be called from a static class function. Why? Because the compiler doesn't know <em>which</em> variable you want. Is it <code>foo.switchChanged</code> or <code>bar.switchChanged</code>?</p>

<hr>

<h1>ISR inside a class as a static class function with static variables</h1>

<p>To make the static function work, it can only access static variables. So we can make switchChanged static. Plus we need to define an instance of this static variable.</p>

<pre class="lang-C++ prettyprint-override"><code>class myClass
  {
  static volatile bool switchChanged;  // declare

  public:

  void begin ()
    {
    pinMode (2, INPUT_PULLUP);
    attachInterrupt (0, switchPressed, CHANGE);
    }  // end of myClass::begin

  static void switchPressed ()
    {
    switchChanged = true;
    }  // end of myClass::switchPressed

  };  // end of class myClass

volatile bool myClass::switchChanged;  // define

myClass foo;  // make an instance of myClass

void setup ()
  {
  foo.begin ();
  }  // end of setup

void loop ()
  {
  // whatever    
  }  // end of loop
</code></pre>

<p>Now the class compiles. However we have thrown away most of the advantages of having a class in the first place, as we are forced to use a static function, and that static function can only access static variables.</p>

<hr>

<h1>Glue routines</h1>

<p>To work around this problem we can write short "glue" routines. These are functions that interface between an ISR and an <strong>instance</strong> of a class.</p>

<pre class="lang-C++ prettyprint-override"><code>class myClass
  {
  volatile bool switchChanged;

  static myClass * instances [2];

  static void switchPressedExt0 ()
    {
    if (myClass::instances [0] != NULL)
      myClass::instances [0]-&gt;switchPressed ();
    }  // end of myClass::switchPressedExt0

  static void switchPressedExt1 ()
    {
    if (myClass::instances [1] != NULL)
      myClass::instances [1]-&gt;switchPressed ();
    }  // end of myClass::switchPressedExt1


  public:

  void begin (const byte whichPin)
    {
    pinMode (whichPin, INPUT_PULLUP);
    switch (whichPin)
      {
      case 2: 
        attachInterrupt (0, switchPressedExt0, CHANGE);
        instances [0] = this;
        break;

      case 3: 
        attachInterrupt (1, switchPressedExt1, CHANGE);
        instances [1] = this;
        break;

      } // end of switch
    }  // end of myClass::begin

  void switchPressed ()
    {
    switchChanged = true; 
    }

  };  // end of class myClass

myClass * myClass::instances [2] = { NULL, NULL };

// instances of our class  
myClass foo; 
myClass bar;

void setup ()
  {
  foo.begin (2);   // pin D2
  bar.begin (3);   // pin D3
  }  // end of setup

void loop ()
  {
  // whatever    
  }  // end of loop
</code></pre>

<p>This is a bit fiddly, however what it is doing is remembering in an array which instance of the class is associated with which interrupt. The "glue" routines switchPressedExt0 and switchPressedExt1 call the appropriate instance of the switchPressed function by using the remembered class pointer.</p>

<p>Now the non-static function switchPressed can access non-static class variables.</p>

<hr>

<p>Adapted from my web page <a href="http://www.gammon.com.au/forum/?id=12983" rel="noreferrer">Calling an ISR from a class</a></p>
