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Suppose we have class Date
,
which does not have a default constructor: there is no good candidate for
a default date. We have a function that returns two dates in form of a boost::tuple
:
boost::tuple<Date, Date> getPeriod();
In other place we want to use the result of getPeriod
,
but want the two dates to be named: begin
and end
. We want to implement
something like 'multiple return values':
Date begin, end; // Error: no default ctor! boost::tie(begin, end) = getPeriod();
The second line works already, this is the capability of Boost.Tuple
library, but the first line won't work. We could set some invented initial
dates, but it is confusing and may be an unacceptable cost, given that these
values will be overwritten in the next line anyway. This is where optional
can help:
boost::optional<Date> begin, end; boost::tie(begin, end) = getPeriod();
It works because inside boost::tie
a
move-assignment from T
is
invoked on optional<T>
,
which internally calls a move-constructor of T
.