const char *v1="testing const char";
char *c = "test char";
String s1="test string";
String *s2;
int i=10;
byte b=12;
//int* i2=20;   //Error
//int *i3=30;   //Error

void setup() {
  // put your setup code here, to run once:
  Serial.begin(115200);
  Serial.println("Hello, ESP32!");
  Serial.println(s1);     //out- test string
  Serial.println(c);      //out- test char
  Serial.println(*c);     //out- t
  //Serial.println(s2);   //error: no matching function for call to 'println(String*&)'
  Serial.println(v1);       //out- testing const char
  Serial.println(*v1);     //out- t
  Serial.println(*(v1+6));  //out- g
  //v1=s1[];    //error: expected primary-expression before ']' token v1=s1[];
  //v1=&s1      //error
  //c=&s1;      //error: cannot convert 'String*' to 'char*'
  //c=s2;       //error: cannot convert 'String*' to 'char*'
  //c=s1[];     //error: expected primary-expression before ']' token c=s1[];
  //c=s1[0];    //error: invalid conversion from 'char' to 'char*' [-fpermissive]
  
    char Buf[50];
    s1.toCharArray(Buf, 50);
    Serial.println(s1);
    Serial.println(Buf);

    const char *cid = s1.c_str();
    //f1(s1);   //error: no matching function for call to 'f1(String&)'
    //f1(&s1);  //error: no matching function for call to 'f1(String*)'
    //f1(*s1);  //error: no match for 'operator*' (operand type is 'String')
    f1(s1.c_str());   //out- test string
    f1(Buf);          //out- test string
    f1(v1);           //out- testing const char
    f1(c);            //out- test char

    //f2(s1.c_str());   //error: invalid conversion from 'const char*' to 'char*' [-fpermissive]
    f2(Buf);          //out- test string
    //f2(v1);           //error: invalid conversion from 'const char*' to 'char*' [-fpermissive]
    f2(c);            //out- test char

    f0(s1);           //out- test string
}

void loop() {
  // put your main code here, to run repeatedly:
  delay(10); // this speeds up the simulation
}

void f0(String s){
  f1(s.c_str());
}

void f1(const char* sn){
  Serial.println(sn);
}

void f2(char* sn){
  Serial.println(sn);
}