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I am just learning an Arduino so this is totally educational purpose call.

I want to instantiate PubSubClient client with WiFiClient parameter.


#include <WiFiClient.h>
#include <PubSubClient.h>

WiFiClient *wifi = NULL;
PubSubClient client(wifi);

void setup() {
  // put your setup code here, to run once:
}

void loop() {
  // put your main code here, to run repeatedly:
}

compilation gives me back:

Compilation error: no matching function for call to /home/tepo/Arduino/libraries/PubSubClient/src/PubSubClient.h:116:4: note:   candidate expects 0 arguments, 1 provided
/home/tepo/Arduino/libraries/PubSubClient/src/PubSubClient.h:88:7: note: candidate: 'PubSubClient::PubSubClient(const PubSubClient&)'
 class PubSubClient : public Print {
       ^~~~~~~~~~~~
/home/tepo/Arduino/libraries/PubSubClient/src/PubSubClient.h:88:7: note:   no known conversion for argument 1 from 'WiFiClient*' to 'const PubSubClient&'

exit status 1

Compilation error: no matching function for call to 'PubSubClient::PubSubClient(WiFiClient*&)'

and listing all the other constructors.

type chain seems to hold: class WiFiClient : public ESPLwIPClient -> class ESPLwIPClient : public Client -> class Client: public Stream

PubSubClient ctor:

 PubSubClient(Client& client);

so why is it not accepting a WifiClient as a parameter?

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2
  • PubSubClient client(*wifi);. it will compile, but it will just crash in runtime since there will be no WiFiClient object at address 0 Commented Nov 17, 2023 at 15:37
  • look at the example code in the library Commented Nov 17, 2023 at 15:43

1 Answer 1

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The issue you're encountering with the PubSubClient constructor in Arduino is related to how you're passing the WiFiClient object. You're currently using a pointer (WiFiClient *wifi), but the PubSubClient constructor expects a reference to a Client object, not a pointer.

The WiFiClient class inherits from the Client, so it can be used with PubSubClient. However, you need to pass an instance of WiFiClient, not a pointer to an instance.

Here's how you can correct your code:

#include <WiFiClient.h>
#include <PubSubClient.h>

WiFiClient wifi;
PubSubClient client(wifi);

void setup() {
  // put your setup code here, to run once:
}

void loop() {
  // put your main code here, to run repeatedly:
}

In this corrected version, wifi is an instance of WiFiClient, not a pointer. When you pass wifi to the PubSubClient constructor, it correctly matches the expected Client& client parameter type.

In C++ a pointer (WiFiClient *) is a variable that holds the memory address of an object. A reference (WiFiClient &) is an alias for an object. The PubSubClient constructor expects a reference because it needs to interact with the Client object throughout its lifecycle, and references are generally safer and more straightforward to use in this context than pointers.

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2
  • yeah, that got it working; however I'm failing to understand where the actual instance of 'wifi' is comming from as I'm not constructing int, rather in the sketch I am using WifiManager that seems 'injecting' it to the declared variable. (I come from java-spring world) That's why I thought pointer will do in this case; it seems a bit weird to me it just "gets supplied" somehow.. Commented Nov 18, 2023 at 21:48
  • What do you mean you're not constructing it? WiFiClient wifi; constructs it. Unlike Java you don't need to do a new procedurally to instantiate a class variable. Commented Nov 19, 2023 at 7:37

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