Only create extractors on first row/col request.

This avoids the overhead of extractor creation when the NumericMatrix is not
going to be queried from JS, e.g., because it's an intermediate. The downside
is that the row/col getters are no longer thread-safe, but they can only
be called from JS and that should be single-threaded anyway.

src/NumericMatrix.cpp

@@ -3,9 +3,9 @@
 
 NumericMatrix::NumericMatrix() {}
 
-NumericMatrix::NumericMatrix(const tatami::NumericMatrix* p) : NumericMatrix(std::shared_ptr<const tatami::NumericMatrix>(p)) {}
+NumericMatrix::NumericMatrix(const tatami::NumericMatrix* p) : ptr(p) {}
 
-NumericMatrix::NumericMatrix(std::shared_ptr<const tatami::NumericMatrix> p) : ptr(std::move(p)), by_row(ptr->dense_row()), by_column(ptr->dense_column()) {}
+NumericMatrix::NumericMatrix(std::shared_ptr<const tatami::NumericMatrix> p) : ptr(std::move(p)) {}
 
 template<class Vector_>
 tatami::NumericMatrix* create_NumericMatrix(int nr, int nc, Vector_ vec, bool colmajor) {
@@ -18,8 +18,8 @@ tatami::NumericMatrix* create_NumericMatrix(int nr, int nc, Vector_ vec, bool co
 
 void NumericMatrix::reset_ptr(std::shared_ptr<const tatami::NumericMatrix> p) {
     ptr = std::move(p);
-    by_row = ptr->dense_row();
-    by_column = ptr->dense_column();
+    by_row.reset();
+    by_column.reset();
 }
 
 NumericMatrix::NumericMatrix(int nr, int nc, uintptr_t values, bool colmajor, bool copy) {
@@ -40,14 +40,20 @@ int NumericMatrix::ncol() const {
     return ptr->ncol();
 }
 
-void NumericMatrix::row(int r, uintptr_t values) const {
+void NumericMatrix::row(int r, uintptr_t values) {
     double* buffer = reinterpret_cast<double*>(values);
+    if (!by_row) {
+        by_row = ptr->dense_row();
+    }
     by_row->fetch_copy(r, buffer);
     return;
 }
 
-void NumericMatrix::column(int c, uintptr_t values) const {
+void NumericMatrix::column(int c, uintptr_t values) {
     double* buffer = reinterpret_cast<double*>(values);
+    if (!by_column) {
+        by_column = ptr->dense_column();
+    }
     by_column->fetch_copy(c, buffer);
     return;
 }

src/NumericMatrix.h

@@ -20,14 +20,17 @@ struct NumericMatrix {
     int ncol() const;
 
 public:
-    void row(int r, uintptr_t values) const;
-
-    void column(int c, uintptr_t values) const;
-
     bool sparse() const;
 
     NumericMatrix clone() const;
 
+    // Not thread-safe! by_row and by_column are initialized
+    // on demand when particular rows and columns are requested
+    // in Javascript. Don't use these functions from C++.
+    void row(int r, uintptr_t values);
+
+    void column(int c, uintptr_t values);
+
 public:
     std::shared_ptr<const tatami::NumericMatrix> ptr;