Compiler warning when "delete"ing a client instance

[fredlcore]

The following code results in the warning below:

max_cul = new WiFiSpiClient();
[...]
delete max_cul;
max_cul = NULL;

warning: deleting object of polymorphic class type 'WiFiSpiClient' which has non-virtual destructor might cause undefined behavior [-Wdelete-non-virtual-dtor]
     delete max_cul;

Is there a way to prevent this warning?

[JiriBilek]

It seems to me an explicit virtual destructor is needed. The destructor should close the connection and free the socket, if used.
I'll fix it ASAP.
Thanks.

[fredlcore]

That would be great, I just added an empty function

WiFiSpiClient::~WiFiSpiClient() {
}

in WiFiSpiClient.cpp as well as

    virtual ~WiFiSpiClient();

in WiFiSpiClient.h and it prevents the warning (not sure if "virtual" is correct here), but you are right, the connection should be closed and the socket freed as well. Thanks for looking into this, looking for the patch!

[JAndrassy]

It seems to me an explicit virtual destructor is needed. The destructor should close the connection and free the socket, if used.
I'll fix it ASAP.
Thanks.

no. the WiFiClient of Arduino net API should not close the connection if destroyed. (server.available() returns a copy, some function can take it in parameter as a copy)

WiFiClient should be only a pointer to internal representation. In ESP8266WiFi library the WiFiClient does it with ClientContext. All objects of the WiFiClient class for the same TCP connection must see the same state of the connection including for example buffer and count of data available.

I recently found out that the WiFiClientSecure for BearSSL in esp8266 WiFi library doesn't work on this principle and the developers fixed it immediately with a not trivial change.

In this library the WiFiClient should only point to WiFiClient object on firmware side as it is in Ethernet library and other WiFi libraries which are only remote API of firmware.

[JiriBilek]

Thanks Juraj!
I see, the problem is really in the server code, thanks for mentioning it. Closing the connection when destroying the WiFiClient object would hopefully be ok and desirable when using ESP as a web client but disastrous when using it as a server.

WiFiSpi/src/WiFiSpiServer.cpp

Line 62 in ec4a9d4

WiFiSpiClient client(_sock);

But it seems to me, there should be a housekeeping after destroying the WiFiClient that is not linked to the server socket as there would be no chance to connect to the socket and close it afterwards.

[fredlcore]

Thanks, that seems to correspond with what I found here:
https://stackoverflow.com/questions/12994920/how-to-delete-an-object-of-a-polymorphic-class-type-that-has-no-virtual-destruct

Does "server code" here mean the "WiFiSpiEsp" code/library? Or code inside the ESP8266-SDK?
In the meantime, should I then refrain from using "delete" or just ignore the warning if the necessary changes are not trivial and will take some time?

[JiriBilek]

The library can be used to run a TCP client or server.
The server opens a listener and every time a connection is made it creates a WiFiClient object. This object is not supposed to free the socket in the WiFiSpiClass::_state array because it serves for subsequent requests.
On the other hand, I think, closing the connection when WiFiClient goes out of scope or is deleted is perfectly fine for client connection (imagine e.g. sending data to a web service using REST API). I have always used the WiFiClient object as static object, so I did not have the problem. I'll fix it once I make a test case.

[JAndrassy]

Jiri, this is my PR to esp8266 WiFi library, which contains a discussion about correct Arduino Server and Client implementation
esp8266/Arduino#7612

[JiriBilek]

Juraj, the problem with multiple WiFiClient objects for one connection is constrained to TCP server functionality, right?
I admit, I am a little confused, must study some code and refresh my code as well.

[JAndrassy]

it is valid for arduino code to copy the Ethernet/WiFiClient object. as assignment or as function parameter by copy (parameter type without & or *). this is not (ab)used widely, but can occur. I can't now find an example. I checked some libraries which work with Client and they all use Client&.
This is similar how String is used.

String someFns(String s) {
  String s2 = s;
  return s2;
}

is valid and works. it looks crazy for a C++ programmer and is not efficient (compiler optimizes it to some level)

[JiriBilek]

I see. The problem is quite bigger than I originally thought.
Even in my code, the implicit copy constructor is used (on return from the function):

WiFiSpiClient WiFiSpiServer::available(uint8_t* status)
{
    WiFiSpiClient client(_sock);

    uint8_t _client_status = client.status();  // creates Client object on ESP side if there is established connection
//    uint8_t _server_status = this->status();  removed, may be related with the comment below, running fine without it 

    if (status != NULL)
        *status = _client_status;

    // TODO: If server is not in listen state, restart it (code present in original WiFiServer.h). I think it is useless.

    if (_client_status == ESTABLISHED)
        return client;

    return WiFiSpiClient(SOCK_NOT_AVAIL);  // closed Client
}

Definitely, WiFiClient destructor should not close the connection.
Thanks for the help.

[JiriBilek]

index to an array on the ESP side. See https://github.com/JiriBilek/WiFiSpiESP/blob/018349de89ef63ff8f4d01792f9bfda29b4c9eca/WiFiSPIESP/WiFiSPICmd.h#L39 and next few lines

It must be synchronized between the protocol (SPI) client and server.

Edit: maybe you are asking the meaning of the command WiFiSpiClient client(_sock); It creates the client object for the given server one.

[JAndrassy]

sorry I deleted the comment to not open a new problem. I don't want to look into source code of your library now, but shouldn't the firmware return 'sock' index of the current client. there can be more then one client connected.

and next problem is that, the esp8266 WiFiServer available() implementation doesn't comply with Arduino.
https://www.arduino.cc/en/Reference/WiFiServerAvailable

[JiriBilek]

@fredlcore : weird thing: the following code compiles without error. Am I missing something?

Compiled with arm-none-eabi-gcc\4.8.3-2014q1

#include "WiFiSpiClient.h"

void setup() {
    WiFiSpiClient *pp = new WiFiSpiClient();
    delete pp;
}

void loop() {
}