functors.tex

\subsection[()]{Function objects}

\begin{frame}[fragile]
  \frametitlecpp[98]{Function objects}
  \begin{block}{Concept}
    \begin{itemize}
    \item also known as functors (no relation to functors in math)
    \item a class that implements \cppinline{operator()}
    \item allows to use objects in place of functions
    \item with constructors and data members
    \end{itemize}
  \end{block}
  \begin{cppcode}
    struct Adder {
      int m_increment;
      Adder(int increment) : m_increment(increment) {}
      int operator()(int a) { return a + m_increment; }
    };
    Adder inc1{1}, inc10{10};
    int i = 3;
    int j = inc1(i);  // 4
    int k = inc10(i); // 13
    int l = Adder{25}(i); // 28
  \end{cppcode}
\end{frame}

\begin{frame}[fragile]
  \frametitlecpp[20]{Function objects}
  \begin{exampleblockGB}{Function objects as function arguments}{https://godbolt.org/z/zxqYG6xzT}{function objects}
    \begin{cppcode}
    int count_if(const auto& range, auto predicate) {
      int count = 0;             // ↑ template (later)
      for (const auto& e : range)
        if (predicate(e)) count++;
      return count;
    }
    struct IsBetween {
      int lower, upper;
      bool operator()(int value) const {
        return lower < value && value < upper;
      }
    };
    int arr[]{1, 2, 3, 4, 5, 6, 7};
    std::cout << count_if(arr, IsBetween{2, 6}); // 3
    // prefer: std::ranges::count_if
    \end{cppcode}
  \end{exampleblockGB}
\end{frame}