updates README

Author: attwoodn

README.md

@@ -134,7 +134,11 @@ auto* expr_raw = make_expr(&my_type::my_bool, op::equals, true);
 delete expr_raw;
 ```
 
-// TODO document the three make_expr functions
+The `make_expr` helper function is templated and overloaded to allow for maximum compatibility for use within expressions. There are three definitions of `make_expr`:
+ * One that accepts a reference to a value-type class member variable, an operator function, and a comparison value (whose type matches the given class member variable);
+ * One that accepts a reference to a pointer-type class member variable, an operator function, and a pointer to a comparison value (whose type matches the given class member variable); and
+ * One that accepts a reference to a const class member function, an operator function, and a comparison value (whose type matches the return type of the given const class member function)
+ 
 
 Please see the section below for more information about expression tree nodes.
 

include/attwoodn/expression_tree.hpp

@@ -384,15 +384,15 @@ namespace attwoodn::expression_tree {
      * Makes an expression tree leaf node for comparing value-type member variables of a class/struct
     */
     template<typename Obj, typename CompValue, typename Op = typename type_id<bool (*)(CompValue, CompValue)>::type>
-    node::expression_tree_leaf_node<Obj, Op, CompValue>* make_expr( const CompValue Obj::* member_var, Op op, CompValue comp_value ) {
+    node::expression_tree_leaf_node<Obj, Op, CompValue>* make_expr( const CompValue Obj::* member_var, Op op, const CompValue comp_value ) {
         return new node::expression_tree_leaf_node<Obj, Op, CompValue>( member_var, op, comp_value );
     }
 
     /**
      * Makes an expression tree leaf node for comparing the return value from a class/struct's const member function
     */
     template<typename Obj, typename CompValue, typename Op = typename type_id<bool (*)(CompValue, CompValue)>::type>
-    node::expression_tree_leaf_node<Obj, Op, CompValue>* make_expr( CompValue (Obj::* member_func)() const, Op op, CompValue comp_value ) {
+    node::expression_tree_leaf_node<Obj, Op, CompValue>* make_expr( CompValue (Obj::* member_func)() const, Op op, const CompValue comp_value ) {
         return new node::expression_tree_leaf_node<Obj, Op, CompValue>( member_func, op, comp_value );
     }