A class is a service if it is publicly and unambiguously derived from execution_context::service, or if it is publicly and unambiguously derived from another service. For a service S, S::key_type shall be valid and denote a type (C++Std [temp.deduct]), is_base_of_v<typename S::key_type, S> shall be true, and S shall satisfy the Destructible requirements (C++Std [destructible]).

The first parameter of all service constructors shall be an lvalue reference to execution_context. This parameter denotes the execution_context object that represents a set of services, of which the service object will be a member. [Note: These constructors may be called by the make_service function. —end note]

A service shall provide an explicit constructor with a single parameter of lvalue reference to execution_context. [Note: This constructor may be called by the use_service function. —end note]

class my_service : public execution_context::service
{
public:
  typedef my_service key_type;
  explicit my_service(execution_context& ctx);
  my_service(execution_context& ctx, int some_value);
private:
  virtual void shutdown() noexcept override;
  ...
};

A service's shutdown member function shall destroy all copies of user-defined function objects that are held by the service.