198. Basic Lambda Callback

The syntax looks like this: [Captures] (Parameters) { Body }

• [ ] Capture Clause: "What variables from the outside world do I need?"

• ( ) Parameters: "What arguments does the caller pass me?" (Just like a normal function).

• { } Body: The code to execute.

int main() {
    int threshold = 50;

    // A Lambda stored in a variable 'check'
    // It captures 'threshold' so it can use it inside.
    auto check = [threshold](int value) {
        if (value > threshold) {
            printf("Alert! Value %d > %d\n", value, threshold);
        }
    };

    check(60); // Prints "Alert! Value 60 > 50"
}

This is where Lambdas beat Function Pointers.

• [ ] (Empty): Captures nothing. Behaves exactly like a standard C function.

• [=] (By Value): Copies all used local variables into the lambda. Safe, but read-only by default.

• [&] (By Reference): Accesses the actual variables. Dangerous if the lambda runs after the variable scope ends.

• [this]: Captures the current object. Essential for using member variables inside a callback.

class Button {
    int id = 1;
public:
    void onClick() {
        // Capture 'this' to access 'id' inside the lambda
        auto callback = [this]() {
            printf("Button %d Clicked\n", this->id);
        };
        callback();
    }
};

How do you store a Lambda? This is the tricky part in firmware.

A. The "Captureless" Optimization (Zero Cost)

If a lambda has empty captures [], the compiler treats it identical to a raw function pointer.

• Cost: Zero RAM overhead. No Heap.

• Use: Compatible with legacy C drivers expecting void (*ptr)(int).

// Legacy C Driver
void register_irq( void (*cb)(int) );

void setup() {
    // Works perfectly! Decays to function pointer.
    register_irq( [](int code) { 
        printf("IRQ %d\n", code); 
    }); 
}

B. The "Capturing" Challenge (std::function)

If a lambda has captures (e.g., [x]), it is no longer just code; it is an object with data. It won't fit in a raw function pointer.

• Solution 1 (Standard): Use std::function<void(int)>.

  • Warning: This often uses malloc (Heap) to store the captured variables. Risky in strict embedded.

• Solution 2 (Template): The "Zero Cost" way for C++.

// Template approach: The compiler generates a custom type for the lambda.
// No Heap. No Virtual calls. Maximum speed.
template <typename Callback>
void run_task(Callback cb) {
    cb();
}

int main() {
    int x = 10;
    // We can capture 'x' without worrying about malloc
    run_task( [x](){ printf("%d", x); } );
}

1. Asynchronous Event Handling

Instead of writing 10 different tiny functions (handleButton1, handleButton2), you define the logic inline during initialization.

button1.onPress( [](){ led.on(); } );
button2.onPress( [](){ motor.stop(); } );

2. Custom Iterators

Running logic over a collection of sensors.

// 'sensors' is an array of objects
std::for_each(sensors.begin(), sensors.end(), [](Sensor& s) {
    s.calibrate(); // Runs for every sensor
});

3. Scoped Locks (RAII)

You can use a lambda to define a critical section that automatically executes logic.

execute_atomic( []() {
    // Interrupts are disabled automatically by the wrapper
    critical_variable++;
}); // Interrupts re-enabled here

Summary Checklist

• Use [] (empty) if you need compatibility with C function pointers.

• Use [this] if writing a lambda inside a class method.

• Avoid [&] (reference capture) for asynchronous tasks (Timers/ISRs).

• Prefer Templates over std::function to avoid Heap usage.