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The String object created as return from IPAddress.toString() as parameter to constructor of NtpClient is temporary. It contains the char array returned by c_str(). NTPClient doesn't copy the string, only stores a reference to it. And the referenced string (char array) doesn't exist at the time the NTPClient wants to use it.

class TestClass {
public:
  TestClass(const char* _ip) : ip(_ip) { }

  void test() {
    Serial.println("--");
    Serial.println(ip);
    Serial.println("--");
  }

private:
  const char* ip;
};


//String s("abc");
//TestClass Test(s.c_str()); <- this works

TestClass Test(String("abc").c_str()); // <- this cant't work

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

The String object created as return from IPAddress.toString() as parameter to constructor of NtpClient is temporary. It contains the char array returned by c_str(). NTPClient doesn't copy the string, only stores a reference to it. And the referenced string (char array) doesn't exist at the time the NTPClient wants to use it.

class TestClass {
public:
  TestClass(const char* _ip) : ip(_ip) { }

  void test() {
    Serial.println("--");
    Serial.println(ip);
    Serial.println("--");
  }

private:
  const char* ip;
};


//String s("abc");
//TestClass Test(s.c_str()); <- this works

TestClass Test(String("abc").c_str()); // <- this can't work

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

The String object created as return from toString() as parameter to constructor of NtpClient is temporary. It contains the char array returned by c_str(). NTPClient doesn't copy the string, only stores a reference to it. And the referenced string (char array) doesn't exist at the time the NTPClient wants to use it.

class TestClass {
public:
  TestClass(const char* _ip) : ip(_ip) { }

  void test() {
    Serial.println("--");
    Serial.println(ip);
    Serial.println("--");
  }

private:
  const char* ip;
};


//String s("abc");
//TestClass Test(s.c_str()); <- this works

TestClass Test(String("abc").c_str()); // <- this cant't work

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

The String object created as return from IPAddress.toString() as parameter to constructor of NtpClient is temporary. It contains the char array returned by c_str(). NTPClient doesn't copy the string, only stores a reference to it. And the referenced string (char array) doesn't exist at the time the NTPClient wants to use it.

class TestClass {
public:
  TestClass(const char* _ip) : ip(_ip) { }

  void test() {
    Serial.println("--");
    Serial.println(ip);
    Serial.println("--");
  }

private:
  const char* ip;
};


//String s("abc");
//TestClass Test(s.c_str()); <- this works

TestClass Test(String("abc").c_str()); // <- this cant't work

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

The String object created as return from toString() as parameter to constructor of NtpClient is temporary. It contains the char array returned by c_str(). It doesn't exist at the time the NTPClient wants to use the string obtained it constructor.

class TestClass {
public:
  TestClass(const char* _ip) : ip(_ip) { }

  void test() {
    Serial.println("--");
    Serial.println(ip);
    Serial.println("--");
  }

private:
  const char* ip;
};


//String s("abc");
//TestClass Test(s.c_str()); <- this works

TestClass Test(String("abc").c_str()); // <- this cant't work

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

The String object created as return from toString() as parameter to constructor of NtpClient is temporary. It contains the char array returned by c_str(). NTPClient doesn't copy the string, only stores a reference to it. And the referenced string (char array) doesn't exist at the time the NTPClient wants to use it.

class TestClass {
public:
  TestClass(const char* _ip) : ip(_ip) { }

  void test() {
    Serial.println("--");
    Serial.println(ip);
    Serial.println("--");
  }

private:
  const char* ip;
};


//String s("abc");
//TestClass Test(s.c_str()); <- this works

TestClass Test(String("abc").c_str()); // <- this cant't work

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