Try

Try is a value which could be exception potentially. For a object t, with the type Try<T>, we could consider t is an exception, or value with type T, or nothing.

In async_simple, Try represents a result which could be potentially exception. For example, both the argument of the callback in Lazy::start and the argument of the callback in Future::thenTry are Try.

The idea and implementation of Try comes from Folly.

Try has three state: Nothing, Exception and Value.

The state of a Try object constructed by the default constructor is Nothing. We could use .available() method to query if a Try object is Nothing.

cpp

void foo() {
    Try<int> t;
    assert(!t.available());
    // ...
}

We could use hasError() to judge if there is Exception.

cpp

void foo() {
    Try<int> t(std::make_exception_ptr(std::runtime_error("")));
    assert(t.hasError());
    // ...
}

We could use getException() and value() to get the exception and value from Try. For example:

cpp

void foo() {
    Try<int> t(std::make_exception_ptr(std::runtime_error("")));
    std::exception_ptr e = t.getException()
    // ...
    Try<int> t2(5);
    int res = t2.value();
}

Note that it would throw exception if we use value() to extract the value from a Try object whose state is Exception:

cpp

void foo() {
    Try<int> t(std::make_exception_ptr(std::runtime_error("")));
    int res = t.value(); // Throw Exception
}

Try ​

Try