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eduffy@gmail.com · Guest

Hello -

I've used Loki's Tuple class in the past before, but this is the first
time this problem has come up. I want a tuple that can hold multiple
values of the same type, just like in Alexandrescu's example in his
Modern C++ book. I'm getting an ambiguous base error from the
GenScatterHeirarchy, and using gcc 3.3.4 or icc 8.1, and the latest
Loki sources on the sourceforge.net site.

Here's the code:

$ cat tuple.cpp

#include "loki/HierarchyGenerators.h"
#include "loki/Typelist.h"

int main(int argc, char *argv[])
{
Loki::Tuple<TYPELIST_2(int,int)> X;
Loki::Field<0>(x) = 11;
// Loki::Field<1>(x) = 38;
}

$ g++ tuple.cpp
loki/Reference/HierarchyGenerators.h: In instantiation of
`Loki::GenScatterHierarchy<Loki::Typelist Loki::NullType> >, Loki::TupleUnit>':
tuple.cpp:7: instantiated from `Loki::Tuple<Loki::Typelist Loki::Typelist >'
tuple.cpp:7: instantiated from here
loki/Reference/HierarchyGenerators.h:62: warning: direct base `
Loki::GenScatterHierarchy<int, Loki::TupleUnit>' inaccessible in `
Loki::GenScatterHierarchy<Loki::Typelist Loki::NullType> >, Loki::TupleUnit>' due to ambiguity
loki/Reference/HierarchyGenerators.h: In static member function `static
typename Loki::Select<Loki::FieldHelper Loki::Select<Loki::FieldHelper H::TList::Head,
typename H::Rebind<typename H::TList::Head>::Result>::Result,
typename
Loki::Select<Loki::FieldHelper H::TList::Head,
typename H::Rebind<typename
H::TList::Head>::Result>::Result>::Result&
Loki::FieldHelper<H, 0>: Very Happy

o(H&) [with H =
Loki::Tuple<Loki::Typelist Loki::Typelist >]':
loki/Reference/HierarchyGenerators.h:205: instantiated from `typename
Loki::FieldHelper<H, i>::ResultType& Loki::Field(H&) [with int i = 0, H
= Loki::Tuple<Loki::Typelist

majic · Guest

After a little more investigation, Andrei made a post on this bug on
the loki sourceforge site:
http://sourceforge.net/tracker/index.php?func=detail&aid=635065&group_id=29557&atid=396644

Field template function error
This error is related to bug #544055.

When using the Field template function with a
GenScatterHierarchy having the same type more than
once the LeftBase type can be ambiguous.

The following error was reported by the Comeau 3.4.01
compiler (using a VC7 back end) and the Reference Loki
implementation when compiling the Point3D Tuple
example from "Modern C++ Desgn" (Section 3.13.2
Generating Tuples - page 70):

base class "Loki::GenScatterHierarchy<int,
Loki::TupleUnit>" is
ambiguous
LeftBase& leftBase = obj;

In fact this error can be predicted from looking at Figure
3.4 from "Modern C++ Desgn". The LeftBase type
GenScatterHierarchy<int, Holder> appears twice in the
hierarchy, so the compiler has a problem deciding which
base is ment when trying to obtain a base reference of
this type from an object of the WidgetInfo type.

A fix would be to add an integer template parameter to
GenScatterHierarchy - defaulting to 0 - which is passed
as is to the left base class and passed incremented by
one to the left side viz (apologies for wrapped lines):

template <class TList, template Idx=0>
class GenScatterHierarchy;

template <class T1, class T2, template int Idx>
class GenScatterHierarchy<Typelist
: public GenScatterHierarchy<T1, Unit, Idx>
, public GenScatterHierarchy<T2, Unit, Idx+1>
{
public:
typedef Typelist<T1, T2> TList;
typedef GenScatterHierarchy<T1, Unit, Idx> LeftBase;
typedef GenScatterHierarchy<T2, Unit, Idx+1> RightBase;
template <typename T> struct Rebind
{
typedef Unit<T> Result;
};
};

template <class AtomicType, template Idx>
class GenScatterHierarchy : public Unit<AtomicType>
{
typedef Unit<AtomicType> LeftBase;
template <typename T> struct Rebind
{
typedef Unit<T> Result;
};
};

template <template
class GenScatterHierarchy<NullType, Unit, Idx>
{
template <typename T> struct Rebind
{
typedef Unit<T> Result;
};
};

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Carl Barron · Guest

In article <1104103459.504544.46560 (AT) z14g2000cwz (DOT) googlegroups.com>,
majic <majic (AT) theRollos (DOT) com> wrote:

Larry Evans · Guest

On 12/27/2004 03:21 AM, majic wrote:
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Larry Evans · Guest

On 12/28/2004 03:42 AM, Larry Evans wrote:

Larry Evans · Guest

On 01/17/2005 01:05 PM, Larry Evans wrote:
[snip]