C++ syntax Problem : how function is pass as parameter

Question

This question already has an answer here: question from:https://stackoverflow.com/questions/65560234/c-syntax-problem-how-function-is-pass-as-parameter

Answer 1

std::accumulate() function is used to accumulate all the values in the range [first, last] both inclusive to any variable initial_sum.

int myfunction (int x, int y) {
  return x+2*y;
}

int main () {
  int init = 100;
  int numbers[] = {10, 20, 30};
  std::cout << std::accumulate (numbers, numbers+3, init, myfunction);

}

and 4th parameter where you are confused is lambda function of c++ is same as usual function but has no name.

[count = unordered_map<int, int> {}] (auto a, auto b) mutable {
    return a + count[b]++;
};