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 (削除) not supported by the Arduino environment (削除ここまで). (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...

• KIIV: Not completely true. It's not supported by avr-g++ compiler. However compilers for ARM based arduinos usually have full support of STL libraries including std::function
• Edgar Bonet: @KIIV: thanks for the heads up. I edited the answer.
• the busybee: You can emulate a closure by an instance of a helper class that stores the captured values and implements the function. I didn't try it, but I expect it to work even for AVRs.