C++ higher order functions [closed]

Question

I a started learning some C++ and don't understand how higher order functions work in C++. Can someone explain c++ 11 functions of higher order on a simple example? I can't find much information on this topic anywhere online.

Answer 1

Many of the standard C++ functions in the <algorithm> header are examples of higher-order functions.

For example the count_if function takes a unary predicate which is a type of callable function and returns a count of objects that match the given predicate. Since count_if is a function which takes another function as an argument, this makes it a higher-order function.

This example doesn't make use of any C++11 features, but C++11 just augments the existing support for higher-order functions in previous C++ standards:

#include <algorithm>
#include <iostream>
#include <vector>

bool is_even(int i) {
  return i % 2 == 0;
}

int main(int argc, char *argv[]) {
  std::vector<int> v;
  for (int i = 0; i < 10; ++i) {
    v.push_back(i);
  }
  std::cout
    << "count = "
    << std::count_if(v.begin(), v.end(), &is_even)
    << std::endl;

  return 0;
}

Converting this into an example that uses some C++11 features is fairly trivial:

#include <algorithm>
#include <iostream>
#include <vector>

int main(int argc, char *argv[]) {
  std::vector<int> v = { 0, 1, 2, 3, 4, 5 };

  std::cout
    << "count = "
    << std::count_if(v.begin(),
                     v.end(),
                     [](int i) -> bool { return i % 2 == 0; })
    << std::endl;

  return 0;
}

In the second example, I changed the vector initialization to use list initialization, and I've replaced is_even with a lambda expression.