Re: [eigen] Feature proposal - add error string to help debug const/non-const mapping errors

[Gael Guennebaud <gael.guennebaud@xxxxxxxxx>]

Is the standard error message that bad? Here is with gcc:

ll.cpp: In function 'int main()':
ll.cpp:58:32: error: invalid conversion from 'const double*' to
'Eigen::Map<Eigen::Matrix<double, -1, -1> >::PointerArgType {aka
double*}' [-fpermissive]
In file included from .../Eigen/Core:299:0,
                 from .../Eigen/SparseCore:4,
                 from .../Eigen/Sparse:17,
                 from ll.cpp:3:
..../Eigen/src/Core/Map.h:164:12: error:   initializing argument 1 of
'Eigen::Map<MatrixType, MapOptions,
StrideType>::Map(Eigen::Map<MatrixType, MapOptions,
StrideType>::PointerArgType, Eigen::Map<MatrixType, MapOptions,
StrideType>::Index, Eigen::Map<MatrixType, MapOptions,
StrideType>::Index, const StrideType&) [with PlainObjectType =
Eigen::Matrix<double, -1, -1>; int MapOptions = 0; StrideType =
Eigen::Stride<0, 0>; Eigen::Map<MatrixType, MapOptions,
StrideType>::PointerArgType = double*; Eigen::Map<MatrixType,
MapOptions, StrideType>::Index = long int]' [-fpermissive]

The first line tell me exactly what's wrong (conversion from const
double* to double*).



With clang:

ll.cpp:58:19: error: no matching constructor for initialization of
'Map<MatrixXd>'
    Map<MatrixXd> m(ptr, 10, 10);
                  ^ ~~~~~~~~~~~
..../Eigen/src/Core/Map.h:164:12: note: candidate constructor not
viable: 1st argument ('const double *') would lose const qualifier
    inline Map(PointerArgType dataPtr, Index nbRows, Index nbCols,
const StrideType& a_stride = StrideType())
           ^
..../Eigen/src/Core/Map.h:151:12: note: candidate constructor not
viable: 1st argument ('const double *') would lose const qualifier
    inline Map(PointerArgType dataPtr, Index a_size, const StrideType&
a_stride = StrideType())
           ^
..../Eigen/src/Core/Map.h:139:12: note: candidate constructor not
viable: requires at most 2 arguments, but 3 were provided
    inline Map(PointerArgType dataPtr, const StrideType& a_stride =
StrideType())
           ^
..../Eigen/src/Core/Map.h:104:79: note: candidate constructor (the
implicit copy constructor) not viable: requires 1 argument, but 3 were
provided
template<typename PlainObjectType, int MapOptions, typename
StrideType> class Map


and more generally, when someone write a class with a member like this:

struct Bar {
  void foo(double* ptr);
};

nobody add a member: void foo(const double*) {  static_assert(false,
"blabla"); }

So it seems to me that such change would go a bit too far, and I'm not
really sure it will ease understanding the problem.

Finally, if you use MatrixXd::Map(ptr, rows, cols), then no
const/non-const issue anymore!

gael

On Mon, Mar 4, 2013 at 5:54 AM, Rohit Garg <rpg.314@xxxxxxxxx> wrote:
> On Sun, Mar 3, 2013 at 7:06 AM, Christoph Hertzberg
> <chtz@xxxxxxxxxxxxxxxxxxxxxxxx> wrote:
>> On 03.03.2013 00:58, Rohit Garg wrote:
>>>
>>> Hi all,
>>>
>>> I have been using the Map objects a lot lately, and I have seen that
>>> often, a simple mistake of mapping a const pointer as, say, MatrixXd.
>>> Obviously, this leads to a big compiler error, which is often
>>> difficult to decipher. I propose that a small error string be added to
>>> Map object, which will aid debugging. Something similar to
>>> "you_mixed_different_types_use_cast_function".
>>>
>>> This could prove very helpful. I know 3.2 beta is being planned, just
>>> how big a change is this?
>>
>>
>> I guess that's a rather small change (basically provide a const Scalar*
>> constructor which has a static assertion).
>> But I would vote against suggesting to "use_cast" in the error string.
>> Maybe something like expecting_a_non_const_pointer or
>> check_your_const_correctness, though this might be also a bit confusing to
>> the novice (but it would be easier to provide a FAQ entry for that).
>
> It's good to know that it's a small change. I mentioned the cast
> string just to make a better reference to what I was talking about. I
> should have phrased it better. Your suggestion for the error string is
> a good one.
>
>
>>
>> Christoph
>>
>>
>>
>>
>> --
>> ----------------------------------------------
>> Dipl.-Inf., Dipl.-Math. Christoph Hertzberg
>> Cartesium 0.049
>> Universität Bremen
>> Enrique-Schmidt-Straße 5
>> 28359 Bremen
>>
>> Tel: +49 (421) 218-64252
>> ----------------------------------------------
>>
>>
>
>
>
> --
> Rohit Garg
>
> http://rpg-314.blogspot.com/
>
> Graduate Student
> Applied and Engineering Physics
> Cornell University
>
>