Re: [sigc] callbacks with std::auto_ptr by value

[Ulrich Eckhardt <eckhardt satorlaser com>]

On Thursday 31 August 2006 22:18, klaus triendl wrote:
> The following code won't compile:
[...]
> void test(std::auto_ptr<int>)
[...]
> int main()
> {
>    std::auto_ptr<int> p;
>    sigc::ptr_fun(&test)(p);

Yes, and that is good. Point is that sigC++ is a 1:n signalling framework. 
Now, imagine that you have two functions called with the same auto_ptr:

   std::auto_ptr<int> p;
   test1(p);
   test2(p);

The result is obviously not going to be what it seems to look like, except to 
someone that knows the semantics of auto_ptr and the function signature. 
Since sigC++ is a simple signal-routing framework, modification of the signal 
(i.e. in this case the auto_ptr) shouldn't take place. If you really want it 
to happen, you should explicitly pass a (non-const) reference to an auto_ptr 
instead. 

This is partially my opinion, but I think this runs analog to the fact that 
auto_ptr is simply not copyable - the (abused) copy constructor only 
implements move semantics.

> If function arguments are passed by value then sigc internally creates a
> const std::auto_ptr& with the type_trait<std::auto_ptr>::take, but
> std::auto_ptr has a copy constructor that takes a nonconst argument:
> auto_ptr& operator =(auto_ptr& nonconst);

This is exactly the point, but even though it technically is a copy 
constructor, it doesn't make auto_ptr copyable, which is a requirement for 
passing it to several slots by value.

my 2cc

Uli

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