How to add functions to the Portenta H7 standard library (Serial)

Hardware Portenta Portenta H7
1

Hi, I am trying to add a function called counter() to Serial.h.
I have done a lot of research and implemented the following, but I get an error.
I have tried to find a way to deal with the error but I am stuck.
Please give me some advice.

Descriptions in the standard library are omitted except for the counter() function. This is because no changes have been made.

Error Description
{path of the ino file}/sketch.ino: : undefined reference to `arduino::UART::counter()'
collect2.exe: error: ld returned 1 exit status

sketch.ino

#include "Serial.h"
UART *cuart;

void setup() {
Serial.begin(BAUD_RATE, SERIAL_8E1);

}

void loop(){
unsigned long value;

value = cuart->counter();

Serial.println(value);
}

Serial.h
...

class UART : public HardwareSerial {
	public:
		UART(int tx, int rx, int rts = -1, int cts = -1);
		UART(PinName tx, PinName rx, PinName rts = NC, PinName cts = NC) : _tx(tx), _rx(rx), _rts(rts), _cts(cts) {}
		UART() {
			is_usb = true;
		}
		void begin(unsigned long);
		void begin(unsigned long baudrate, uint16_t config);
		void begin(unsigned long baudrate, uint16_t config, bool no_rx_pullup);
		void end();
		int available(void);
		int peek(void);
		int read(void);
		void flush(void);
		size_t write(uint8_t c);
		size_t write(const uint8_t*, size_t);
		using Print::write; // pull in write(str) and write(buf, size) from Print
		operator bool();
		operator mbed::FileHandle*();	// exposes the internal mbed object
		unsigned long counter(void);

....

Serial.cpp
...

unsigned long UART::counter(void){
	return 3;
}

...