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++ 2014 [temp.deduct]), is_base_of_v<typename S::key_type, S> shall be true, and S shall satisfy the Destructible requirements (C++ 2014 [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 can 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 can be called by the use_service function. — end note ]
[ Example:
class my_service : public execution_context::service{
public:
using key_type = my_service;
explicit my_service(execution_context& ctx);
my_service(execution_context& ctx, int some_value);
private:
virtual void shutdown() noexcept override;
...
};
— end example ]
A service's shutdown member function shall destroy all copies of function objects that are held by the service.