Added a to_string method to instructions

This way if you are not using an ostream you don't have to create one
just to get the string. Replaced uses in show_goto_function_json and
show_goto_functions_xml

Since this isn't performance critical we use std::string as makes the
result connect more cleanly to external APIs like the JSON and XML
libraries.

Author: thk123

src/goto-programs/goto_program_template.h

@@ -20,6 +20,8 @@ Author: Daniel Kroening, kroening@kroening.com
 #include <util/symbol_table.h>
 #include <util/source_location.h>
 #include <util/std_expr.h>
+#include <sstream>
+#include <string>
 
 typedef enum { NO_INSTRUCTION_TYPE=0,
                GOTO=1,          // branch, possibly guarded
@@ -240,6 +242,13 @@ class goto_program_templatet
 
       return false;
     }
+
+    std::string to_string() const
+    {
+      std::ostringstream instruction_id_builder;
+      instruction_id_builder << type;
+      return instruction_id_builder.str();
+    }
   };
 
   typedef std::list<class instructiont> instructionst;

src/goto-programs/show_goto_functions_json.cpp

@@ -77,11 +77,8 @@ json_objectt show_goto_functions_jsont::get_goto_functions(
       {
         json_objectt instruction_entry=json_objectt();
 
-        std::ostringstream instruction_id_builder;
-        instruction_id_builder << instruction.type;
-
         instruction_entry["instructionId"]=
-          json_stringt(instruction_id_builder.str());
+          json_stringt(instruction.to_string());
 
         if(instruction.code.source_location().is_not_nil())
         {

src/goto-programs/show_goto_functions_xml.cpp

@@ -88,11 +88,8 @@ xmlt show_goto_functions_xmlt::get_goto_functions(
       {
         xmlt &instruction_entry=xml_instructions.new_element("instruction");
 
-        std::ostringstream instruction_id_builder;
-        instruction_id_builder << instruction.type;
-
         instruction_entry.set_attribute(
-          "instructionId", instruction_id_builder.str());
+          "instructionId", instruction.to_string());
 
         if(instruction.code.source_location().is_not_nil())
         {