[Rcpp-devel] typedef and const

Douglas Bates bates at stat.wisc.edu
Thu May 20 15:07:28 CEST 2010

In the process of converting code to use the Rcpp classes I discovered
that a combination of typedef and const does not work as I expected it
to work.

The background is that the C-based sparse matrix library called
CHOLMOD (used in the Matrix package for R) declares a number of C
structs such as cholmod_sparse, cholmod_dense, ...  Most of the
CHOLMOD functions take pointers to these arguments and return pointers
to freshly-allocated structures.  For brevity I declared typedefs like

typedef cholmod_sparse *CHM_SP;

I had thought that I could replace an argument declaration like

const cholmod_sparse *A


const CHM_SP A

but discovered after much experimentation that const CHM_SP A expands to

cholmod_sparse *A

In other words, I can't use const to modify a typedef as I expected.
Can someone explain why?  Is there an alternative declaration that
accomplishes what I want?  For the time being I have reverted to the
full declaration of const cholmod_sparse*

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