Re: [boost] iterator_adaptors

From: John E. Potter (jpotter_at_[hidden])
Date: 2000-12-31 20:44:37

On Fri, 29 Dec 2000, Arun Sivaramakrishnan wrote:

> Jeremy wrote:
> > 
> > If you want to use a function returning a reference, use projection
> > iterator instead of transform.
> 
> Same problem.

Not really.  Transform has value semantics and projection has reference
semantics.

> Since we cannot have pointer to references or reference to references
> there are compiler errors when AdaptableUnaryFunction 
> returns a reference type.
> As Dave said, we do need specializations to overcome this problem
> atleast for some compilers.

This is a lack of documentation.  Projection_iterator has reference
semantics always.  There is a general problem with reference template
parameters.  Vector<int&>?  Set<int&>?  They just don't work.  But,

struct RefFunc : public unary_function<SomeThing, int> {
   int& operator() (SomeThing& st) { return st.someIntField; }
   };

works fine with both std::unary_function and projection_iterator.  It
is a lie which works well.  The only purpose of std::unary_function is
to extract the correct value_type.  Reference and pointer are always
value_type& and value_type*.  The function object must have reference
semantics.

Improvements welcome.  It could make transform and projection one
iterator.

John