Optional callback in C++

void_cb.cpp

/**
 * Functor behaving like an unset function pointer (nullptr) with 
 * arbitrary number of arguments.
 *
 * Usage:
 * ```
 * #include "Utils/void_cb.hpp"
 *
 * template <typename CB=void_cb>
 * void f(CB callback=nullptr) {
 *   // ...
 *   if (callback != nullptr) {
 *     callback("whatever arguments your callback takes");
 *   }
 *   // ...
 * }
 * ```
 */
struct void_cb {
    constexpr void_cb(void* = nullptr) {}

    template <typename... T>
    void operator()(T...) { assert(false); };

    constexpr operator void*() const { return nullptr; }
};