I am trying to develop a NTP client library for ESP8266/Arduino to make adding NTP sync an easier task.

Basically, I've thought about a constructor as NTPClient(String host, int interval); and a NTPClient.begin() function to register sync function.

My problem is with that registering.

I have a public function: time_t ntpClient::getNtpTime() that connects to NTP server, decodes the response and returns a time_t variable.

Then, inside boolean ntpClient::begin() function I try to run setSyncProvider(ntpClient::getNtpTime). I've also tried to use setSyncProvider(this->getNtpTime).

I get a compile error here:

ntpClient.cpp:In member function 'boolean ntpClient::begin()
ntpClient.cpp:79:39: error: cannot convert 'ntpClient::getNtpTime' from type 'time_t (ntpClient::)() {aka long int (ntpClient::)()}' to type 'getExternalTime {aka long int (*)()}
Error compiling project sources

What could I do to allow registering this function as sync provider to hide NTP internals to my sketches?

  • 1
    Perhaps you should show us your code...?
    – Majenko
    Commented Dec 18, 2015 at 23:00

1 Answer 1


What could I do to allow registering this function as sync provider to hide NTP internals to my sketches?

The function should be a static member function. A "normal" member function does not have the right prototype as there is a hidden parameter ("this").

static time_t getNtpTime();

To solve this you need a single-ton (an instance of the client class) that the static function can use.

class NTPClient {
   NTPClient() ...
     s_client = this;
   time_t getNtpTime();
   static time_t getNtpTime() ( return (s_client->getNtpTime()); }
   static NTPClient* s_client;
NTPClient::s_client = NULL;
NTPClient ntpClient;

Alternatively a global function and a known instance:

class NTPClient { ... };
extern NTPClient ntpClient;
time_t getNtpTime() { return (ntpClient.getNtpTime()); } 


  • It was that, thank you. I've tried to change getNtpTime() to static but doing that makes impossible to access variables and non static funcions. So, the only possibility I see is adding a static funcion in main program that wraps a non static ntpClient.getNtpTime(). I'm not an expert C++ programmer at all. Do you think there is any other possibility? I'll upload my code to github and publish here the link.
    – gmag11
    Commented Dec 20, 2015 at 17:16
  • That is what the last line was all about. You need an instance that the static member function may use. For instance, add a static ntpClient pointer to the class and assign it in the constructor. The static member function can then use that to call the implementation. Commented Dec 20, 2015 at 17:44
  • I've uploaded my code to https://github.com/gmag11/NtpClient. I'll check the link about single-ton design pattern.
    – gmag11
    Commented Dec 22, 2015 at 12:24
  • Please see the update code example above. Cheers! Commented Dec 22, 2015 at 12:51
  • I got it working finally. I've done sync provider registering in setup() function. I will try to add a begin member function in a later moment. Last version is on GitHub. Thank you for your recommendation about singleton.
    – gmag11
    Commented Dec 22, 2015 at 23:32

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