Allow passing number of rows from outside

NEWS.md

@@ -1,5 +1,9 @@
 # cpp11 (development version)
 
+* New `writable::data_frame` constructor that also takes the number of rows as
+  input. This accounts for the edge case where the input list has 0 columns but
+  you'd still like to specify a known number of rows (#272).
+
 * `cpp11::writable::r_vector<T>::iterator` no longer implicitly deletes its
   copy assignment operator (#360).
 

cpp11test/src/test-data_frame.cpp

@@ -43,6 +43,28 @@ context("data_frame-C++") {
     expect_true(df.nrow() == 10);
   }
 
+  test_that("writable::data_frame::nrow works with 0x0 dfs") {
+    SEXP x = PROTECT(Rf_allocVector(VECSXP, 0));
+
+    cpp11::writable::data_frame df(x);
+    expect_true(df.nrow() == 0);
+
+    UNPROTECT(1);
+  }
+
+  test_that("writable::data_frame::nrow works with 10x0 dfs (#272)") {
+    SEXP x = PROTECT(Rf_allocVector(VECSXP, 0));
+
+    bool is_altrep = false;
+    int nrow = 10;
+
+    // Manually specify `nrow` using special constructor
+    cpp11::writable::data_frame df(x, is_altrep, nrow);
+    expect_true(df.nrow() == 10);
+
+    UNPROTECT(1);
+  }
+
   test_that("writable::data_frame works") {
     using namespace cpp11::literals;
     cpp11::writable::data_frame df({"x"_nm = {1, 2, 3}, "y"_nm = {"a", "b", "c"}});

inst/include/cpp11/data_frame.hpp

@@ -67,7 +67,11 @@ namespace writable {
 class data_frame : public cpp11::data_frame {
  private:
   writable::list set_data_frame_attributes(writable::list&& x) {
-    x.attr(R_RowNamesSymbol) = {NA_INTEGER, -static_cast<int>(calc_nrow(x))};
+    return set_data_frame_attributes(std::move(x), calc_nrow(x));
+  }
+
+  writable::list set_data_frame_attributes(writable::list&& x, int nrow) {
+    x.attr(R_RowNamesSymbol) = {NA_INTEGER, -nrow};
     x.attr(R_ClassSymbol) = "data.frame";
     return std::move(x);
   }
@@ -76,6 +80,8 @@ class data_frame : public cpp11::data_frame {
   data_frame(const SEXP data) : cpp11::data_frame(set_data_frame_attributes(data)) {}
   data_frame(const SEXP data, bool is_altrep)
       : cpp11::data_frame(set_data_frame_attributes(data), is_altrep) {}
+  data_frame(const SEXP data, bool is_altrep, int nrow)
+      : cpp11::data_frame(set_data_frame_attributes(data, nrow), is_altrep) {}
   data_frame(std::initializer_list<list> il)
       : cpp11::data_frame(set_data_frame_attributes(writable::list(il))) {}
   data_frame(std::initializer_list<named_arg> il)