[Rd] Checking multiple inheritance of S4 objects using R's C API

[Simon Urbanek]

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 using 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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