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Re: Limit Debug mode impact: overload __niter_base

On 01/07/18 21:20 +0200, François Dumont wrote:
    Here is a new proposal between yours and mine.

    It is still adding a function to wrap what __niter_base unwrap, I called it __nwrap_iter for this reason. But it takes advantage of

Since "niter" refers to __normal_iterator I think a name based on
"niter" would be better than nsomething_iter.

__niter_lift (misuse of functional programming term?)
__niter_raise (misuse of linear algebra term?)

knowing that __niter_base will only unwrap random access iterator to use an expression to that will do the right thing, no matter the original iterator type.

OK, since __niter_base only transforms types based on __normal_iterator that seems safe to assume (in theory we could use
__normal_iterator with non-random access iterators, but we don't).

Could you please add a comment to the __nwrap_iter saying something

 // Reverse the __niter_base transformation to get a
 // __normal_iterator back again (this assumes that __normal_iterator
 // is only used to wrap random access iterators, like pointers).

    I eventually found no issue in the testsuite, despite the std::equal change. I might have had a local invalid test.

Yes, I *did* test it already :-)

diff --git a/libstdc++-v3/include/bits/stl_algobase.h b/libstdc++-v3/include/bits/stl_algobase.h
index d429943..003ae8d 100644
--- a/libstdc++-v3/include/bits/stl_algobase.h
+++ b/libstdc++-v3/include/bits/stl_algobase.h
    __niter_base(_Iterator __it)
    { return __it; }

+  // Convert an iterator of type _From to an iterator of type _To.
+  // e.g. from int* to __normal_iterator<int*, Seq>.
+  template<typename _Iterator>
+    inline _Iterator
+    __nwrap_iter(_Iterator, _Iterator, _Iterator __res)
+    { return __res; }
+  template<typename _From, typename _To>
+    inline _From
+    __nwrap_iter(_From __from, _To __to, _To __res)
+    { return __from + (__res - __to); }

Every time you call this function you pass it:

 __nwrap_iter(x, __niter_base(x), y)

So can the __niter_base(x) call happen inside __nwrap_iter?


 template<typename _From, typename _To>
   inline _From
   __nwrap_iter(_From __from, _To __res)
   { return __from + (__res - __niter_base(__from)); }

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