What are closures in C++ and how do you use them?

Question

C++ / C#

Good afternoon! I can’t understand what closures are for and why it’s better to live with them than without them? Who uses it in real projects, can you share your experience?

Answer 1

Closures are inseparable from lambdas. They allow you to pass references to the lambda. For example, in a lambda you needed to change a private class variable. Without a closure, you would have to make a setter and a getter (which already changes the class interface, they can be called where it would not be worth it), pass a pointer to an object into the lambda.
And so you work with an object variable inside the lambda, as if it were a local variable of the lambda.

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

using namespace std;

using IntV = vector<int>;

int main(int argc, char const *argv[]) {

  IntV v = {0, 9, 4, 6, 7, 8, 5, 6};
  size_t count = 0;

  for_each(begin(v), end(v), [](IntV::value_type x){cout << x << " ";});
  cout << endl;

  sort(begin(v), end(v), [&count /*closure*/](IntV::value_type &a, IntV::value_type &b){
    ++count;
    return a < b;
  });

  cout << count << endl;

  for_each(begin(v), end(v), [](IntV::value_type x){cout << x << " ";});
  cout << endl;

  return 0;
}

Here is an example for you to count the number of comparisons when sorting a vector.