[Rcpp-devel] Compiler error I can't understand

[Douglas Bates]

On Sat, May 8, 2010 at 4:31 AM, Romain Francois
<romain at r-enthusiasts.com> wrote:
> Hi,
>
> Without seeing the actual code, it looks like a candidate for const_cast.
> See for example: http://www.cplusplus.com/doc/tutorial/typecasting/
>
> If you indeed create a stripped down example, I'll look into it.

Thanks, Romain.  I had reached a similar conclusion myself last
evening, thinking that I needed a cast.

I did get it working but not with the casts that I expected.  In the
end I used a C-like cast, i.e. (const CHM_SP)this because neither a
static_cast<const CHM_SP>(this) nor a const_cast<CHM_SP>(this) seemed
to work.

Thanks again for the suggestion.

> Le 08/05/10 00:20, Douglas Bates a écrit :
>>
>> The lme4a package (available only from R-forge at present) uses
>> facilities from the Matrix package and Rcpp.  The Matrix package
>> itself uses the CHOLMOD sparse matrix package written in C.  In the C
>> code the sparse matrix representations are defined as structs and
>> passed as pointers to such structs.  In lme4a I have source files
>> MatrixNs.h and MatrixNs.cpp that provide thin class wrappers around
>> these structs.  So the Matrix package header files define
>>
>> #ifdef __cplusplus
>> #extern "C" {
>> #endif
>>
>> typedef struct cholmod_sparse_struct {
>>   /* various members */
>> } cholmod_sparse;
>>
>> typedef struct cholmod_common_struct {
>>  /* various members */
>> } cholmod_common;
>>
>> typedef cholmod_sparse *CHM_SP;
>> typedef cholmod_common *CHM_CM;
>>
>> CHM_SP M_cholmod_transpose(const CHM_SP, int, CHM_CM);
>>
>> #ifdef __cplusplus
>> }
>> #endif
>>
>> In MatrixNs.h I declare
>>
>> cholmod_common c;
>> class chmSp : cholmod_sparse {
>> public:
>>    chmSp(Rcpp::S4);
>>    CHM_SP transpose(int) const;
>> }
>>
>> with the definition in MatrixNs.cpp of
>>
>> CHM_SP chmSp::transpose(int values) const {
>>    return M_cholmod_transpose(this, values,&c);
>> }
>>
>> and consistently get it thrown back at me by the compiler because of
>> an illegal transformation from "const cholmod_sparse*" to
>> "cholmod_sparse*" in the line where I call M_cholmod_transpose.
>>
>> This is severely impeding the development of the code because I can't
>> pass chmSp objects as const chmSp&  (or, apparently equivalently, chmSp
>> const&) which could cause a lot of unnecessary copying.  Of course,
>> those sparse matrices are members of larger objects, etc. and right
>> now there is the possibility that everything is being copied
>> unnecessarily.
>>
>> Should this be happening and if so, why?  I believe that the
>> declaration of M_cholmod_transpose in effect for this piece of code
>> has the first argument declared as const CHM_SP.  (As I said, the
>> original code did not have const in the declaration and I may have
>> only modified one of the declarations and somehow it is picking up
>> another.  As a last resort I have copied all the relevant files from
>> the Matrix installed package area to the lme4a source area in my local
>> copy just so I am certain of the declaration that is in effect.)
>>
>> Is there something peculiar about using the wrapper class in the sense
>> that "this" is a pointer to a chmSp and I am using it as a
>> cholmod_sparse pointer?  Alternatively, is it something about using
>> the "this" pointer itself?
>>
>> The compiler errors are of the form
>>
>> MatrixNs.cpp: In member function ‘cholmod_sparse*
>> MatrixNs::chmSp::transpose(int) const’:
>> MatrixNs.cpp:286: error: invalid conversion from ‘const
>> cholmod_sparse*’ to ‘cholmod_sparse*’
>>
>> Any suggestions would be gratefully accepted - especially if they
>> solve the problem :-)
>>
>> If someone wants to look at the actual source files I will try to
>> create a stripped down example.  The version on R-forge doesn't show
>> exactly this structure because I don't want to leave a version up
>> there that doesn't compile.
>> _______________________________________________
>> Rcpp-devel mailing list
>> Rcpp-devel at lists.r-forge.r-project.org
>> https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/rcpp-devel
>>
>>
>
>
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> Romain Francois
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