Editing Function object (section)

=== Maintaining state ===

Another advantage of function objects is their ability to maintain a state that affects <code>operator()</code> between calls. For example, the following code defines a [[generator (computer science)|generator]] counting from 10 upwards and is invoked 11 times.

<syntaxhighlight lang="cpp">
#include <algorithm>
#include <iostream>
#include <iterator>

class CountFrom {
 public:
  CountFrom(int count) : count_(count) {}
  
  int operator()() { return count_++; }

 private:
  int count_;
};

int main() {
  const int state(10);
  std::generate_n(std::ostream_iterator<int>(std::cout, "\n"), 11,
                  CountFrom(state));
}
</syntaxhighlight>

In C++14 or later, the example above could be rewritten as:

<syntaxhighlight lang="cpp">
#include <algorithm>
#include <iostream>
#include <iterator>

int main() {
  std::generate_n(std::ostream_iterator<int>(std::cout, "\n"), 11,
                  [count=10]() mutable { return count++; });
}
</syntaxhighlight>