Using lambdas as callback functions

Question

Using the Ticker library for the ESP32, how can I use a lambda as an argument to the attach method?

tickerSystemManager.attach(1000, [&](){
  systemManager->sync();
});

Attempting to use the above, I get the following error:

Matrx.cpp:11:4: error: no matching function for call to 'Ticker::attach(int, Matrx::setup()::<lambda()>)'
...
Ticker.h:40:8: note: candidate: void Ticker::attach(float, Ticker::callback_t)

How can I capp a lambda into this method?

Answer 1

As @timemage explains in a comment, you cannot pass a capturing lambda to a function that expects a plain pointer to function. There is, however, a way out of this: the Ticker class provides an overload of the attach() method that allows you to provide a parameter of any type to tour callback:

template<typename TArg>
void attach(float seconds, void (*callback)(TArg), TArg arg)

You can use this version of attach() and provide it both

• a non-capturing lambda that gets a pointer to systemManager
• that pointer as a third argument.

Edit: I did a few tests, and it seems my compiler cannot properly perform template argument deduction in this case, but the approach works if you explicitly specialize the template. Namely:

tickerSystemManager.attach<typeof systemManager>(
    1000,
    [](typeof systemManager p){ p->sync(); },
    systemManager);