Arduino: Custom library produces strange behavior

I am fairly new to arduino as well as C++. I want to make a plant watering system. I have found this tutorial to measure soil moisture. It's in Czech, but you can find a piece of code to handle the moist sensor. This program runs without any problems. I have decidede to enclose this algorithm to a class so it's more readable to me. I followed arduinos tutorial to create custom library.

MoistSensor.h

#ifndef MoistSensor_h
#define MoistSensor_h

#include "Arduino.h"

class MoistSensor {
  public:
    MoistSensor(int analogPin, int digitalPin, int vccPin);
    void measure();
    int getMoisture();
    void setup();
  private:
    int _analogPin;
    int _digitalPin;
    int _vccPin;
    int _moisture;
    void convertMoisture(int analogValue);
};
#endif

MoistSensor.cpp

#include "MoistSensor.h"
#include "Arduino.h"

MoistSensor::MoistSensor(int analogPin, int digitalPin, int vccPin) {
  _analogPin = analogPin;
  _digitalPin = digitalPin;
  _vccPin = vccPin;

  Serial.println("constructor ok: " + analogPin);
  Serial.println("constructor ok: " + digitalPin);
  Serial.println("constructor ok: " + vccPin);
};

int MoistSensor::getMoisture() {
  return _moisture;
};

void MoistSensor::measure() {
  digitalWrite(_vccPin, HIGH);
  delay(100);
  Serial.println("Value: " + analogRead(_analogPin));
  digitalWrite(_vccPin, LOW);
};

void MoistSensor::convertMoisture(int analogValue) {
  if (analogValue <= 230) {
    _moisture = 100;
  }
  else if (analogValue >= 650) {
    _moisture = 0;
  }
  else {
    _moisture = 100 - ((analogValue - 230 ) / 4.2);
  }
};

void MoistSensor::setup() {
  pinMode(_analogPin, INPUT);
  pinMode(_digitalPin, INPUT);
  pinMode(_vccPin, OUTPUT);
  digitalWrite(_vccPin, LOW);
};

Arduino source code:

#include <MoistSensor.h>

#define analogPin A0
#define digitalPin 3
#define vccPin 4

unsigned long time = 0;

MoistSensor sensor(analogPin, digitalPin, vccPin);

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

void loop() {
  if (millis() - time > 3000) {
    
    sensor.measure();
    
    time = millis();
  }
}

The output produced by my program looks like this:

I run in on Arduino NANO on processor ATmega328P (Old bootloader).

I have no idea what could be wrong.