ESP32 Forum

Posted: **Fri Feb 11, 2022 6:39 am**

Is there a proper way to initialise a timer inside a class?

I am trying to do a c++ project and i want to user the high resolution timer.
The class is about XH711 adc 24 bit.

I have an init function where i do everything needed for the adc and i want in there to init also the timer by seting the
esp_timer_create_args_t

When i set the callback function i also set a class function for the callback. I always get error on build

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"void (Xh711::*)(void *args)" cannot be used to initialize an entity of type "esp_timer_cb_t"

and my setup of args is

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    const esp_timer_create_args_t timerArgs={
        .callback = onTimer,
        .arg = NULL,
        .dispatch_method = ESP_TIMER_TASK,
        .name = "Measurement",
        .skip_unhandled_events = false
    };

When i also tried this:

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    const esp_timer_create_args_t timerArgs={
        .callback = Xh711::onTimer,
        .arg = NULL,
        .dispatch_method = ESP_TIMER_TASK,
        .name = "Measurement",
        .skip_unhandled_events = false
    };

and i get error of:

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cannot convert 'Xh711::onTimer' from type 'void (Xh711::)(void*)' to type 'esp_timer_cb_t' {aka 'void (*)(void*)'}

What is the proper way to do this?

Posted: **Fri Feb 11, 2022 9:19 am**

You need to use a static class member, that will match the prototype of the C callback. Static class members don't have a specific object associated with them (no notion of 'this') so you need to use the .arg pointer to pass 'this' to that function; in that static class member function, you can then cast it back to an object and do the things you need to do.

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