calling a method on an uninitialized module object aborts the R session

[kevinushey]

For a module class without a default constructor, calling new(Class) with no arguments deliberately creates an "uninitialized" object: the generated initialize method falls back to cpp_object_dummy() (R/Module.R), which stores a dummy pointer and notes

## like initializer but a dummy for the case of no default
## constructor.  Will throw an error if the object is used.

The C++ side implements that contract in src/module.cpp:

#define CHECK_DUMMY_OBJ(p) if (p == rcpp_dummy_pointer) throw Rcpp::not_initialized()

But the three method-invocation entry points that use it -- CppMethod__invoke, CppMethod__invoke_void, and CppMethod__invoke_notvoid -- are plain .External functions with no BEGIN_RCPP/END_RCPP. The Rcpp::not_initialized exception escapes the entry point into R's C evaluator, where there is no handler, and the process calls std::terminate(): the entire R session aborts. tryCatch() on the R side never gets a chance. The intended behavior is clearly a regular R error ("C++ object not initialized. (Missing default constructor?)").

The XPtr constructor validation throws (not_compatible, null-pointer checks) in the same functions and in class__newInstance can escape the same way, though those require calling the internal entry points with malformed arguments; the dummy-object path is reachable from ordinary, documented usage.

Reproducible example

repro.cpp:

#include <Rcpp.h>

using namespace Rcpp;

class Widget {
public:
    Widget(double v_) : v(v_) {}

    double value() {
        return v;
    }

private:
    double v;
};

RCPP_MODULE(widget_module) {
    class_<Widget>("Widget")
        .constructor<double>()
        .method("value", &Widget::value);
}

repro.R:

Rcpp::sourceCpp("repro.cpp")

# Widget has no default constructor, so new(Widget) with no arguments
# takes the documented cpp_object_dummy() path
w <- methods::new(Widget)
cat("uninitialized object created\n")

# intended: an R error, "C++ object not initialized. (Missing default
# constructor?)"; actual: the C++ exception escapes .External uncaught
res <- tryCatch(w$value(), error = function(e) conditionMessage(e))
cat("caught R error:", res, "\n")

Output with R 4.6.1 (macOS, Rcpp 1.1.2 -- src/module.cpp is unchanged on current master):

uninitialized object created
libc++abi: terminating due to uncaught exception of type Rcpp::not_initialized: C++ object not initialized. (Missing default constructor?)
Abort trap: 6

Rscript exits with code 134 (SIGABRT); the final cat() is never reached.

I'll follow up with a PR that wraps these entry points in BEGIN_RCPP/END_RCPP, so the exception is converted to the intended R error. (This also lets the XPtr locals in those frames destruct properly on the error path instead of being skipped by the exception's escape.)