convert static arrays to dynamic arrays and return, have wrong data.

[Don via Digitalmars-d]

On Sunday, 9 November 2014 at 15:09:10 UTC, H. S. Teoh via 
Digitalmars-d wrote:
> On Sun, Nov 09, 2014 at 08:29:58AM +0000, AlanThinker via 
> Digitalmars-d wrote:
>> It seems that, D's array is strange,
>> It can implicit convert static arrays to dynamic arrays no 
>> error and
>> no warning.
>> But when I return the converted arrays out the function.
>> Outside function will get some wrong data.
>> 
>> It may very easily cause some bug because no error when 
>> convert static
>> arrays to dynamic arrays.
> [...]
>
> Yes, this is a known problem. There may even be an issue filed 
> in
> bugzilla about it (if not, please file one!). The problem is 
> that local
> static arrays are allocated on the stack, and the implicit 
> conversion to
> dynamic array is simply taking a slice of the stack-allocated 
> array. As
> a result, after the function returns, the slice is now pointing 
> at stack
> memory that has gone out of scope.
>
> I'm not sure if the current compiler issues a warning / error 
> if you do
> this in @safe code, but IMO it should do this even in @system 
> code since
> the implicit conversion is almost never correct.
>
>
> T

The problem is, that you need to be able to take a slice of a 
stack-allocked array (otherwise stack allocated arrays are 
useless). Eg you should be able to pass a slice of a stack array 
to writefln().

Detecting if the slice is returned, requires flow analysis. 
Currently the front-end doesn't do any flow analysis at all, 
except for a couple of special cases like closures and super() 
calls.