Default Initialize ExpressionRef to Zero The default constructor of ExpressionRef now creates a compile time constant zero assignment. This patch is required, because a reason change in Jet assumes default 0 initialization. Change-Id: I8185cef587c17ab828896bce1e768170f8229d4e

diff --git a/include/ceres/codegen/internal/expression_ref.h b/include/ceres/codegen/internal/expression_ref.h
index d73d477..5499930 100644
--- a/include/ceres/codegen/internal/expression_ref.h
+++ b/include/ceres/codegen/internal/expression_ref.h

@@ -46,7 +46,7 @@
 //
 // ExpressionRef should be passed by value.
 struct ExpressionRef {
-  ExpressionRef() = default;
+  ExpressionRef() : ExpressionRef(0.0) {}
 
   // Create a compile time constant expression directly from a double value.
   // This is important so that we can write T(3.14) in our code and
@@ -57,6 +57,10 @@
   // must work for T = Jet<ExpressionRef>.
   ExpressionRef(double compile_time_constant);
 
+  // Adds the expression to the active graph and initializes this ExpressionRef
+  // accordingly.
+  ExpressionRef(const Expression& expression);
+
   // By adding this deleted constructor we can detect invalid usage of
   // ExpressionRef. ExpressionRef must only be created from constexpr doubles.
   //
@@ -97,8 +101,6 @@
 
   // The index into the ExpressionGraph data array.
   ExpressionId id = kInvalidExpressionId;
-
-  static ExpressionRef Create(ExpressionId id);
 };
 
 // A helper function which calls 'InsertBack' on the currently active graph.

diff --git a/internal/ceres/codegen/code_generator_test.cc b/internal/ceres/codegen/code_generator_test.cc
index 8b4aafc..f799a75 100644
--- a/internal/ceres/codegen/code_generator_test.cc
+++ b/internal/ceres/codegen/code_generator_test.cc

@@ -46,7 +46,7 @@
   auto code = gen.Generate();
   EXPECT_EQ(code.size(), reference.size());
 
-  for (int i = 0; i < code.size(); ++i) {
+  for (int i = 0; i < std::min(code.size(), reference.size()); ++i) {
     EXPECT_EQ(code[i], reference[i]) << "Invalid Line: " << (i + 1);
   }
 }
@@ -69,7 +69,7 @@
   T d = T(std::numeric_limits<double>::infinity());
   T e = T(-std::numeric_limits<double>::infinity());
   T f = T(std::numeric_limits<double>::quiet_NaN());
-  T g;  // Uninitialized variables should not generate code!
+  T g;  // Uninitialized variables are 0 initialized.
   auto graph = StopRecordingExpressions();
   std::vector<std::string> expected_code = {
       "{",
@@ -79,12 +79,14 @@
       "  double v_3;",
       "  double v_4;",
       "  double v_5;",
+      "  double v_6;",
       "  v_0 = 0;",
       "  v_1 = 123.5;",
       "  v_2 = 2;",
       "  v_3 = std::numeric_limits<double>::infinity();",
       "  v_4 = -std::numeric_limits<double>::infinity();",
       "  v_5 = std::numeric_limits<double>::quiet_NaN();",
+      "  v_6 = 0;",
       "}"};
   GenerateAndCheck(graph, expected_code);
 }

diff --git a/internal/ceres/codegen/expression_ref.cc b/internal/ceres/codegen/expression_ref.cc
index 883d4ea..369aec4 100644
--- a/internal/ceres/codegen/expression_ref.cc
+++ b/internal/ceres/codegen/expression_ref.cc

@@ -37,23 +37,19 @@
 namespace internal {
 
 ExpressionRef AddExpressionToGraph(const Expression& expression) {
+  return ExpressionRef(expression);
+}
+
+ExpressionRef::ExpressionRef(double compile_time_constant)
+    : ExpressionRef(
+          Expression::CreateCompileTimeConstant(compile_time_constant)) {}
+
+ExpressionRef::ExpressionRef(const Expression& expression) {
   ExpressionGraph* graph = GetCurrentExpressionGraph();
   CHECK(graph)
       << "The ExpressionGraph has to be created before using Expressions. This "
          "is achieved by calling ceres::StartRecordingExpressions.";
-  return ExpressionRef::Create(graph->InsertBack(expression));
-}
-
-ExpressionRef ExpressionRef::Create(ExpressionId id) {
-  ExpressionRef ref;
-  ref.id = id;
-  return ref;
-}
-
-ExpressionRef::ExpressionRef(double compile_time_constant) {
-  id = AddExpressionToGraph(
-           Expression::CreateCompileTimeConstant(compile_time_constant))
-           .id;
+  id = graph->InsertBack(expression);
 }
 
 ExpressionRef::ExpressionRef(const ExpressionRef& other) { *this = other; }

diff --git a/internal/ceres/codegen/expression_ref_test.cc b/internal/ceres/codegen/expression_ref_test.cc
index 280043b..c9a118b 100644
--- a/internal/ceres/codegen/expression_ref_test.cc
+++ b/internal/ceres/codegen/expression_ref_test.cc

@@ -48,13 +48,14 @@
   T a = T(0);
   T b = T(123.5);
   T c = T(1 + 1);
-  T d;  // Uninitialized variables should not generate code!
+  T d;  // Uninitialized variables are also compile time constants
   auto graph = StopRecordingExpressions();
 
   ExpressionGraph reference;
   reference.InsertBack(Expression::CreateCompileTimeConstant(0));
   reference.InsertBack(Expression::CreateCompileTimeConstant(123.5));
   reference.InsertBack(Expression::CreateCompileTimeConstant(2));
+  reference.InsertBack(Expression::CreateCompileTimeConstant(0));
   EXPECT_EQ(reference, graph);
 }