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?

• timemage: Lambda expressions that take have no captures generate some implementation type that is implicitly convertible to a basic pointer to function type. Lambda expressions that have a capture group don't qualify for that and are generally speaking not going to be compatible, because in practice what the compiler generates is a class whose members represent the captures, which makes the pointer into a pointer to class member function and therefore incompatible with regular pointer to function.
• Matt Clark: So what can I do to call the sync method of my object? It seems as if every attempt to do so give me some error or another.
• Mat: If systemManager is a global, just remove the & from the capture.

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);

• timemage: You've managed to make this on topic. Might as well go a step further and show the user-data-capable overload in the context of a lambda expression.
• Edgar Bonet: @timemage: Good suggestion. I updated the answer.
• Matt Clark: I really appreciate the answer, this compiled and seems to run as expected! I am still trying to learn Cpp while setting up an ESP project and you are definitely giving me more to look into!