R devel - Feb 2021 - Checking multiple inheritance of S4 objects using R's C API

Hi,

Suppose I have the following two classes in R:


setClass("Person", representation(name = "character", age =
"numeric"))
setClass("Employee", representation(boss = "Person"),
contains = "Person")


I can successfully check inheritance in base R:


employee <- new("Employee", name = "Jack", age = 25)

inherits(employee, "Employee")
#> [1] TRUE
inherits(employee, "Person")
#> [1] TRUE


I could not achieve the same in Rcpp:


Rcpp::cppFunction('
void check_class(Rcpp::S4 x) {
  Rcout << "Class: " <<
as<std::string>(x.attr("class")) << std::endl;
  Rcout << "Rf_inherits(x, \\"Employee\\") : "
<< Rf_inherits(x,
"Employee") << std::endl;
  Rcout << "Rf_inherits(x, \\"Person\\")   : "
<< Rf_inherits(x, "Person")
<< std::endl;
  Rcout << "R_extends : " << R_extends(x,
Rf_mkString("Person"),
R_GlobalEnv) << std::endl;
}')

check_class(employee)
#> Class: Employee
#> Rf_inherits(x, "Employee") : 1
#> Rf_inherits(x, "Person")   : 0
#> R_extends :
#> Error in extends(new("Employee", boss = new("Person",
name character(0), : 'class1' must be the name of a class or a class
definition


I expected `Rf_inherits(x, "Person")` to be true as well.

How can I check whether an object inherits from parent classes in R's C API?

Thanks,
Emre.


P.S. This question originally posted <a href="
https://stackoverflow.com/q/66201929/2275286">here</a>.

	[[alternative HTML version deleted]]

Emre,

inherits() was designed for S3 classes and at C level only ever works for S3
classes.

In S4 world you should use is().

There is no equivalent C-level API for is() so, unfortunately, you likely have
to use Rf_eval() of is(x, "class").

At low-level there is R_S4_extends() which allows you to get the list of
superclasses for S4 objects, but according to the definition of some people it
is not part of the official R API so your mileage may vary.

Cheers,
Simon



> On Feb 16, 2021, at 12:03 PM, Emre Gonulates <egonulates at
gmail.com> wrote:
> 
> Hi,
> 
> Suppose I have the following two classes in R:
> 
> 
> setClass("Person", representation(name = "character",
age = "numeric"))
> setClass("Employee", representation(boss = "Person"),
contains = "Person")
> 
> 
> I can successfully check inheritance in base R:
> 
> 
> employee <- new("Employee", name = "Jack", age = 25)
> 
> inherits(employee, "Employee")
> #> [1] TRUE
> inherits(employee, "Person")
> #> [1] TRUE
> 
> 
> I could not achieve the same in Rcpp:
> 
> 
> Rcpp::cppFunction('
> void check_class(Rcpp::S4 x) {
>  Rcout << "Class: " <<
as<std::string>(x.attr("class")) << std::endl;
>  Rcout << "Rf_inherits(x, \\"Employee\\") : "
<< Rf_inherits(x,
> "Employee") << std::endl;
>  Rcout << "Rf_inherits(x, \\"Person\\")   : "
<< Rf_inherits(x, "Person")
> << std::endl;
>  Rcout << "R_extends : " << R_extends(x,
Rf_mkString("Person"),
> R_GlobalEnv) << std::endl;
> }')
> 
> check_class(employee)
> #> Class: Employee
> #> Rf_inherits(x, "Employee") : 1
> #> Rf_inherits(x, "Person")   : 0
> #> R_extends :
> #> Error in extends(new("Employee", boss =
new("Person", name > character(0), : 'class1' must be the
name of a class or a class definition
> 
> 
> I expected `Rf_inherits(x, "Person")` to be true as well.
> 
> How can I check whether an object inherits from parent classes in R's C
API?
> 
> Thanks,
> Emre.
> 
> 
> P.S. This question originally posted <a href="
> https://stackoverflow.com/q/66201929/2275286">here</a>.
> 
> 	[[alternative HTML version deleted]]
> 
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