Re: valarray help

[Rolf Magnus]

Mephistopheles Jones wrote:

> I'm having trouble when using a valarray as a class member.  I compile
> with g++, and my gcc is version 3.3.5.  Here's a highly simplified
> example:
> 
> struct ValWrapper {
>   valarray<int> item;
> };
> 
> int main() {
>   ValWrapper v;

Here, v.item gets default initialized to a valarray with size 0.

>   v.item=valarray<int>(-1,10);

Here, you assign a valarray with a different size to v.item, which results
in undefined behavior. Both valarrays in an assignment need to be of the
same size.

>   v.item[1]=23;    // <--- segfault here
>   return 0;
> }
> 
> The assignment in the penultimate line of main gives a segfault.  However,
> when I do this
> 
> int main() {
>   valarray<int> v(-1,10);

This creates v as a valarray of size 10. No undefined behavior here.

>   v[1]=23;
>   return 0;
> }
> 
> all works fine.  Replacing all instances of "valarray" with "vector" above
> gives an error as well, but curiously the errors are different.  I get
> "Segmentation fault (core dumped)" for valarrays and "Aborted (core
> dumped)" for vectors.

Are you really just replacing "valarray" with "vector"? No other change? So
you have:

  v.item=vector<int>(-1,10);

? Then it's not surprising. For some reason, the parameters of the valarray
constructor are reversed compared to other containers. So you're trying to
create a vector of size -1 with each entry having a value of 10. The -1
gets converted to an unsigned value of std::vector<int>::size_type, which
results in a very big value. Your system probably can't allocate such a big
amount of memory, and so a bad_alloc exception is thrown, which you don't
catch, so std::unexpected is called, which in turn calls std::terminate,
which calls std::abort. Therefore, your program gets aborted.

> I believe my assignments are syntactically correct. 

Syntactically, yes.

> Any pointers?

No, pointers are not needed ;-)