system_error(error_code ec, const string& what_arg);
Effects: Constructs an object of class system_error.
Postconditions: code() == ec.
string(what()).find(what_arg) != string::npos.
system_error(error_code ec, const char* what_arg);
Effects: Constructs an object of class system_error.
Postconditions: code() == ec.
string(what()).find(what_arg) != string::npos.
system_error(error_code ec);
Effects: Constructs an object of class system_error.
Postconditions: code() == ec.
system_error(int ev, const error_category& ecat, const string& what_arg);
Effects: Constructs an object of class system_error.
Postconditions: code() == error_code(ev, ecat).
string(what()).find(what_arg) != string::npos.
system_error(int ev, const error_category& ecat, const char* what_arg);
Effects: Constructs an object of class system_error.
Postconditions: code() == error_code(ev, ecat).
string(what()).find(what_arg) != string::npos.
system_error(int ev, const error_category& ecat);
Effects: Constructs an object of class system_error.
Postconditions: code() == error_code(ev, ecat).
const error_code& code() const noexcept;
Returns: ec or error_code(ev, ecat), from the constructor,
as appropriate.
const char* what() const noexcept override;
Returns: An ntbs incorporating the arguments supplied in the constructor.
[ Note: The returned ntbs might be the contents of what_arg + ": " +
code.message(). — end note ]