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Subject: Re: [boost] [array] assignement operator
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2009-05-15 08:24:21

Joseph Gauterin wrote:
On Friday, May 15, 2009 7:52 AM
> On Fri, May 15, 2009 at 12:28 PM, vicente.botet
> <vicente.botet_at_[hidden]> wrote:
> > Boost.Array defines an assignement operator with the
> following prototype

[snip expected copy assignment operator]

> > I was wondering if this can be extended to manage with
> arrays of different sizes, having as effect the copying of
> the minimum of N and M elements from the source to the target.
> >
> > template<typename T, std::size_t N>
> > class array {
> > public:
> > /...
> > template<typename U, std::size_t M>
> > array& operator=(const array<U, M>& rhs);
> > // Effects:std::copy(rhs.begin(),rhs.begin()+min(N,M), begin())
> > };
> >
> > Is there anything wrong with this interface?
> I think such an assignment operator is counter-intuitive and its use
> will lead to hard to read code. I'd expect two objects to be
> equivalent after one is assigned to the other.

This is absolutely the case. That's the point of copy assignment operators, std::auto_ptr and a few other classes notwithstanding! For example, Rogue Wave provides a substring class for their string class that uses the copy assignment operator to update the referenced string, not to make the destination substring equivalent to the source. While it provides a convenient interface, it also leads to surprises, particularly for the uninitiated.

> I certainly wouldn't
> expect an array on the LHS to still contain some of its original
> elements after a smaller RHS array is assigned to it.

array is unusual in that the elements begin with indeterminate values when not constructed from an initializer list, and size() always returns N, so its behavior is unusual for a container however you look at it. Retaining the original values is the only possible behavior for such a function when the source is larger than the destination. It would also be surprising to find only a subset of a larger source in the destination when using a copy assignment operator, but array doesn't grow.

The suggested idea may have its uses, but it has sufficiently odd behavior that it ought not to be a member function and certainly not the copy assignment operator. Perhaps a free function is in order.

Rob Stewart robert.stewart_at_[hidden]
Software Engineer, Core Software using std::disclaimer;
Susquehanna International Group, LLP

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