I am trying to make a small keyboard class to handle two buttons(in the future >2) that are connected to my arduino UNO. The being pressed are detected using interrupts. Here is the code:

Timer

class Timer{
  private:
    uint32_t start_time;
  public:
    Timer(){
      start_time=millis();
    }
    void mark(){
      start_time=millis();
    }
    uint32_t lapsed(){
      return millis()-start_time;
    }
};

Keyboard

class KeyboardBtn{
  private:
    const static byte clicksDelay=255;      //255 ms max
    struct btn{
      byte state=0;   //0->nothing;1-click;2-dblClick
      Timer clickDelay;
    };
    volatile static btn buttons[2];         //only two buttons for now
    
  public:
    KeyboardBtn(){
      attachInterrupt(digitalPinToInterrupt(2),this->int_handler_btn0,FALLING);       //Initially the pins are set to HIGH(pullup behaviour)
      attachInterrupt(digitalPinToInterrupt(3),this->int_handler_btn1,FALLING);
    }
    
    static void int_handler_btn0(){
      btns_handler(0);
    }
    static void int_handler_btn1(){
      btns_handler(1);
    }
    static void btns_handler(byte key){
        if(buttons[key].state==1 && buttons[key].clickDelay.lapsed()<=clicksDelay){
          buttons[key].state=2;
        }else{
          buttons[key].state=1;
          buttons[key].clickDelay.mark(); //Pressed one time? mark this and wait for second press
        }
    }
    byte stateChanged(byte key){
      //Function called to get key status
      byte temp_state=buttons[key].state;
      if(temp_state==1 && buttons[key].clickDelay.lapsed()<clicksDelay){
        return 0; //We still wait:don't know if it's a dblClick or single
      }else{
        buttons[key].state=0;
        return temp_state;
      }
    }
};
volatile KeyboardBtn::btn KeyboardBtn::buttons[2];

But the compiler gives me headaches(of course my code is wrong somewhere but he could just have kept quite about it):

C:\Users....\Humidity.ino: In static member function 'static void KeyboardBtn::btns_handler(byte)':

C:\Users...\Humidity\Humidity.ino:74:68: warning: passing 'volatile Timer' as 'this' argument of 'uint32_t Timer::lapsed()' discards qualifiers [-fpermissive]

     if(buttons[key].state==1 && buttons[key].clickDelay.lapsed()<=clicksDelay){

                                                                ^

C:\Users...\Humidity\Humidity.ino:78:40: warning: passing 'volatile Timer' as 'this' argument of 'void Timer::mark()' discards qualifiers [-fpermissive]

       buttons[key].clickDelay.mark();

                                    ^

C:\Users....\Humidity\Humidity.ino: In member function 'byte KeyboardBtn::stateChanged(byte)':

C:\Users....\Humidity\Humidity.ino:83:58: warning: passing 'volatile Timer' as 'this' argument of 'uint32_t Timer::lapsed()' discards qualifiers [-fpermissive]

   if(temp_state==1 && buttons[key].clickDelay.lapsed()<clicksDelay){

                                                      ^

What is the problem? Note:It does allow me to upload the code but I am almost certain it will generate some errors while running...