Is it safe to return a size of a container then changing that container in a finally block?

Question

I'm writing a function that detaches the data from a container and returns the number of elements detached:

int detach(foo f)
{
    try {
        /*some code*/
        return m_ids.size();
    } finally {
        m_ids.clear();
    }
}

foo is an object that receives the data in m_ids, which is a java.util.List.

My concern: is this code safe? Will it return the size of m_ids before it's cleared?

Wouldn't it have been faster to just try and see? ..then Google " return try finally java" and look for official documentation.? — ABuckau, Jul 05 '16 at 10:45
Coming from a C background I'm concerned about the "definedness" of such a construct. — , Jul 05 '16 at 10:45

score 9 · Accepted Answer · answered Jul 05 '16 at 10:43

The code is safe. The return value is established (and stored somewhere) before the finally block is reached. Once the finally block has ran, the stored return value is sent to the function caller.

Note thought that if you had written, in poor coding style, return m_ids.size(); as the last statement in the finally block then 0 would have been returned to the caller: a return value in a finally block overrides any other return value.

score 1 · Answer 2 · answered Jul 05 '16 at 10:53

Check the generated bytecode:

int detach(java.util.List);
    Code:
       0: aload_1
       1: invokeinterface #4,  1  // InterfaceMethod java/util/List.size:()I
       6: istore_2
       7: aload_1
       8: invokeinterface #5,  1  // InterfaceMethod java/util/List.clear:()V
      13: iload_2
      14: ireturn
      15: astore_3
      16: aload_1
      17: invokeinterface #5,  1  // InterfaceMethod java/util/List.clear:()V
      22: aload_3
      23: athrow

istore_2 is the operation that stores the result of size into a local variable, which is then loaded (iload_2) and get returned.

score -5 · Answer 3 · answered Jul 05 '16 at 10:44

-5

Your finally block will never execute if there no exception thrown.

answered Jul 05 '16 at 10:44

Michael Rhöse

117
6

That's wrong. It will _always_ be executed. – Thomas Jul 05 '16 at 10:44
That is incorrect! (You learn something new every day). Try it yourself. – Bathsheba Jul 05 '16 at 10:44

Is it safe to return a size of a container then changing that container in a finally block?

3 Answers3