Class FuncHolder

cpp-subprocess/subprocess.hpp:1033–1038

  };
  template <typename T>
  struct FuncHolder: HolderBase {
    FuncHolder(T func): func_(std::move(func)) {}
    void operator()() const override { func_(); }
    // The function pointer/reference
    T func_;
  };

  std::unique_ptr<HolderBase> holder_ = nullptr;
};