Make Expression type default to constant (#213)

include/sleipnir/autodiff/Expression.hpp

@@ -62,7 +62,7 @@ struct SLEIPNIR_DLLEXPORT Expression {
   IntrusiveSharedPtr<Expression> adjointExpr;
 
   /// Expression argument type.
-  ExpressionType type = ExpressionType::kLinear;
+  ExpressionType type = ExpressionType::kConstant;
 
   /// Either nullary operator with no arguments, unary operator with one
   /// argument, or binary operator with two arguments. This operator is
@@ -104,20 +104,9 @@ struct SLEIPNIR_DLLEXPORT Expression {
   uint32_t refCount = 0;
 
   /**
-   * Type tag used for constructing the "zero" expression.
+   * Default constructor.
    */
-  struct ZeroSingleton_t {};
-
-  /**
-   * Type tag used for constructing the "zero" expression.
-   */
-  static inline ZeroSingleton_t ZeroSingleton;
-
-  /**
-   * Constructs an instance of "zero", which has special meaning in expression
-   * operations. This should only be constructed once via Zero().
-   */
-  explicit Expression(ZeroSingleton_t);
+  Expression() = default;
 
   /**
    * Copy constructor.

src/autodiff/Expression.cpp

@@ -8,12 +8,10 @@
 namespace sleipnir {
 
 IntrusiveSharedPtr<Expression>& Zero() {
-  static auto expr = MakeExpression(Expression::ZeroSingleton);
+  static auto expr = MakeExpression();
   return expr;
 }
 
-Expression::Expression(ZeroSingleton_t) : type{ExpressionType::kConstant} {}
-
 Expression::Expression(double value, ExpressionType type)
     : value{value}, adjointExpr{Zero()}, type{type}, args{Zero(), Zero()} {}