From a36a88bce8e58f2449743b9e235ca93a6ac8dc91 Mon Sep 17 00:00:00 2001
From: Eduard Valeyev <eduard@valeyev.net>
Date: Wed, 24 Jul 2024 13:59:24 -0400
Subject: [PATCH] ttg::matrix::Triplet is read-once by default

---
 examples/ttg_matrix.h | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/examples/ttg_matrix.h b/examples/ttg_matrix.h
index ce563cb4a..e0d6cb3f6 100644
--- a/examples/ttg_matrix.h
+++ b/examples/ttg_matrix.h
@@ -16,10 +16,12 @@ namespace ttg {
 
   namespace matrix {
 
+    enum class ReadOnceTriplet { yes, no };
+
     /// element of a sparse matrix = {row index, col index, value}
 
-    /// movable replacement for Eigen::Triplet
-    template<typename Value, typename StorageIndex=typename Eigen::SparseMatrix<Value>::StorageIndex >
+    /// move-capable replacement for Eigen::Triplet
+    template<typename Value, ReadOnceTriplet ReadOnce = ReadOnceTriplet::yes, typename StorageIndex=typename Eigen::SparseMatrix<Value>::StorageIndex >
     class Triplet {
      public:
       Triplet() = default;
@@ -42,10 +44,15 @@ namespace ttg {
       const StorageIndex& col() const { return m_col; }
 
       /** \returns the value of the element */
-      const Value& value() const { return m_value; }
+      std::conditional_t<ReadOnce == ReadOnceTriplet::yes, Value&&, const Value&> value() const {
+        if constexpr (ReadOnce == ReadOnceTriplet::yes)
+          return std::move(m_value);
+        else
+          return m_value;
+      }
      protected:
       StorageIndex m_row = -1, m_col = -1;
-      Value m_value;
+      mutable Value m_value;
     };
 
     // matrix shape = maps {column,row} index to {row,column} indices