new patch from @lightmare for protecting against expression.to_string…

… misusage - closes #1232

include/mapnik/expression_string.hpp

@@ -36,27 +36,27 @@ MAPNIK_DECL std::string to_expression_string(expr_node const& node);
 
 /*
 The following two templates are intentionally invalid and will prompt
-a compile error if ever instanciated. This should prevent accidentally
+a compile error if ever instantiated. This should prevent accidentally
 passing a pointer (either raw or shared) as the argument.  Without them,
 the compiler could construct a temporary expr_node(bool) using
 implicit pointer-to-bool conversion, thus any non-null pointer
 would yield "true".
 */
 
 template <typename T>
-std::string to_expression_string(T const* x)
+std::string to_expression_string(T const* expr_node_ptr)
 {
-    x = 0;
     throw std::logic_error("to_expression_string() called with pointer argument");
-    return std::string();
+    // compile error intended here; comment on the next line shows in clang output
+    return expr_node_ptr; // to_expression_string() called with pointer argument
 }
 
 template <typename T>
-std::string to_expression_string(boost::shared_ptr<T> const& x)
+std::string to_expression_string(boost::shared_ptr<T> const& expr_node_ptr)
 {
-    x = 0;
     throw std::logic_error("to_expression_string() called with pointer argument");
-    return std::string();
+    // compile error intended here; comment on the next line shows in clang output
+    return expr_node_ptr; // to_expression_string() called with pointer argument
 }
 }