Switch from S4SXP to OBJSXP

[eddelbuettel]

Per the R News RSS feed this morning:

CHANGES IN R-devel C-LEVEL FACILITIES

‘SEXP’ type ‘S4SXP’ has been renamed ‘OBJSXP’ to support experimenting with alternative object systems. The ‘S4SXP’ value can still be used in ‘C’ code but is now deprecated. Based on contriubtions from the R Consortium Object-Oriented Programming Working Group.

I only see two uses in src/api.cpp so should be quick, and we can probably #ifdef this properly to use S4SXP for R < 4.4.0

[eddelbuettel]

Upon closer look it is even only in the type2name() wrapper. So maybe this can be as simple as

#if R_Version >= R_Version(4,4,0)
        case OBJSXP:    return "OBJSXP";
#else
        case S4SXP:     return "S4SXP";
#endif

Can we think of cases where this may hurt an existing package? Maybe more defensize to add the #if but not hide the older version under #else ?

[eddelbuettel]

So maybe

#if R_Version >= R_Version(4,4,0)
        case OBJSXP:    return "OBJSXP"; 		// replaces S4SXP in R 4.4.0 
#endif
        case S4SXP:     return "S4SXP"; 		// for backwards comp. 

I'll check that the compiler does not complain about double entries as both are defined to the same value (25).