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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.