Problems with void r_vector<T>::push_back(const named_arg&)

[honghaoli42]

I'd like to convert a package from Rcpp to cpp11 since we need no more than a data passing interface.

While doing some simple tests, i encountered some problems regarding the method r_vector<T>::push_back(const named_arg&). I haven't been able to test with all types, so I'll only provide examples with cpp11::list.

1. Not really a problem but just a seemingly inconsistent behavior between constructors taking std::initializer_list of SEXP and named_arg:

cpp11::cpp_function('list testCpp11() {
  writable::list res_1({"named"_nm = "value"});  // conversion during the construction of named_arg
  writable::list res_2({as_sexp("unnamed_value")});  // require SEXP
  return res_1;
}')
print(testCpp11())

#> $named
#> [1] "value"

2. When trying to push_back to an initialized list:

# 2.1 push_back an unnamed value to a named list, ok
cpp11::cpp_function('list testCpp11() {
  writable::list named_list({"named"_nm = "value"});
  named_list.push_back(as_sexp("unnamed_value"));
  return named_list;
}')
print(testCpp11())

#> $named
#> [1] "value"

#> [[2]]
#> [1] "unnamed_value"

# 2.2 push_back a named value to a named list, name is missing
cpp11::cpp_function('list testCpp11() {
  writable::list named_list({"named"_nm = "value"});
  named_list.push_back("another_named"_nm = "value");
  return named_list;
}')
print(testCpp11())

#> $named
#> [1] "value"

#> [[2]]
#> [1] "value"

# 2.3 push_back an unnamed value to an unnamed list, ok
cpp11::cpp_function('list testCpp11() {
  writable::list unnamed_list({as_sexp("unnamed_value")});
  unnamed_list.push_back(as_sexp("unnamed_value"));
  return unnamed_list;
}')
print(testCpp11())

#> [[1]]
#> [1] "unnamed_value"

#> [[2]]
#> [1] "unnamed_value"

# 2.4 push_back a named value to an unnamed list, error
cpp11::cpp_function('list testCpp11() {
  writable::list unnamed_list({as_sexp("unnamed_value")});
  unnamed_list.push_back("named"_nm = "value");
  return unnamed_list;
}')
print(testCpp11())

#> Error in testCpp11() :
#>   Invalid input type, expected 'character' actual 'NULL'

3. When trying to push_back to an empty list:

# 3.1 push_back an unnamed value to an empty list, ok
cpp11::cpp_function('list testCpp11() {
  writable::list empty_list;
  empty_list.push_back(as_sexp("unnamed_value"));
  return empty_list;
}')
print(testCpp11())

#> [[1]]
#> [1] "unnamed_value"

# 3.2 push_back a named value to an empty list, error
cpp11::cpp_function('list testCpp11() {
  writable::list empty_list;
  empty_list.push_back("named"_nm = "value");
  return empty_list;
}')
print(testCpp11())

#> Error in testCpp11() :
#>   Invalid input type, expected 'character' actual 'NULL'

My questions are:

1. Are the results of test 2.2, 2.4, 3.2 intended behavior ?
2. Concerning what we actually need in our package, is there a way to push_back a named value to a named list without losing the value name (as shown in 2.2) ?

Also I'd like to request a feature to enable as_sexp for Container<Container<T>> (maybe with the conversion to list of list).

Thank you !

[honghaoli42]

Hi @jimhester , thanks for the quick fix for 2. and 3.. As for 1., with a closer look there are actually some problems with the interaction between the class named_arg and push_back for other types. I'll open another issue for that.

Now i'd like to ask for your thoughts about list as_sexp(Container<Container<T>>);. Should I open a feature request issue for that, or is it not going to be considered for now ?

Thanks in advance.

[jimhester]

I think it is out of scope, but if you want us to consider it you would need to open a new issue.