2
Return values are future values:
•Calling an active object method is
nonblocking, but a return value (and “out” arguments) cannot be used until an explicit
wait for the future to materialize.
active class Calc {
public: double TotalOrders() { return ComputeSomething(); }
};
public: double TotalOrders() { return ComputeSomething(); }
};
//
in calling code, using a Calc object
future<double> tot = calc.TotalOrders(); // call is nonblocking
… potentially lots of work … // parallel work
DoSomethingWith( tot.wait() ); // explicitly wait to accept
future<double> tot = calc.TotalOrders(); // call is nonblocking
… potentially lots of work … // parallel work
DoSomethingWith( tot.wait() ); // explicitly wait to accept
Why require explicit wait? Four compelling reasons:
•No silent loss of concurrency (e.g.,
early “logFile << tot;”).
•Explicit block point for writing into
lent objects (“out” args).
•Explicit point for emitting
exceptions.
•Need to be able to pass futures onward
to other code (e.g., DoSomethingWith( tot ) ≠ DoSomethingWith( tot.wait() )).
Futures