Output contract's arguements

[monarch_dodra]

On Thursday, 19 September 2013 at 13:02:15 UTC, Peter Alexander 
wrote:
> On Thursday, 19 September 2013 at 12:45:01 UTC, Joseph Rushton 
> Wakeling wrote:
>> On 19/09/13 14:19, Peter Alexander wrote:
>>> You mean a separate copy from v?
>>>
>>> But surely then the copy would *always* be the same in the 
>>> in-contract as it is
>>> in the out-contract? What's the point?
>>
>> It makes sense to be able to access v.old (or v.onEntry or 
>> whatever you want to call it) in the out contract, as then you 
>> could do some checks on the final value of v that are 
>> dependent on its entry value.
>
> Yes that makes sense, but I have no idea what the difference 
> between the input and output contracts are in the original 
> post. What errors will the output v != 0 check catch that the 
> input v != 0 test not catch if they both use the input value of 
> v?

That was a reduced example to show that v was modified. It 
prevented running the actual output contract which was:

size_t msb(T)(T v)
in
{
   assert(v != 0);
}
out(ret)
{
   assert(2^^ret <= v);
   assert(v < 2^^(ret + 1));
}
body
{
   size_t ret = 0;
   while(v >>= 1) ++ret;
   return ret;
}
//----

I want to calculate the msb, and in the out contract, I want to 
validate it is correct, by comparing it with what the input was.

In this case, this doesn't really work, since the input was 
"consumed".