How do I use lambda functions in Arduino?

Question

I want to pass a lambda function as an argument to a method. Example:

T reduce(const T initial, const std::function<T(T, T)> acc) {
    T value = initial;
    for (size_t i = 0; i < S; ++i) {
        value = acc(value, this->values[i]);
    }

    return value;
}

...

int sum = this->reduce(42, [](int acc, int value) { return acc + value; });

If I try to compile this code for Arduino, I get an obvious error:

error: 'function' in namespace 'std' does not name a template type

How can I use a lambda function in Arduino?

Answer 1

std::function is not supported by the Arduino environment. (Edit: as mentioned by KIIV in a comment, it's supported on ARM targets, but not on AVR.) You can, however, pass a non-capturing lambda to a function that expects a plain function pointer. For example, given

T reduce(const T initial, T (*acc)(T, T)) {
    T value = initial;
    for (size_t i = 0; i < S; ++i) {
        value = acc(value, this->values[i]);
    }
    return value;
}

You can call it as

int sum = test.reduce(42, [](int acc, int value){ return acc + value; });

If you want your lambda to capture something, it would seem you are out of luck...