Better error messages when dereferencing (e.g.) a seq iterator as if it were a map iterator

[GoogleCodeExporter]

When parsing a yaml file such as

---
title: Home
items:
  - Key:
      SubKeyOne: [] #sequence of integers, in the order they are dependent
      SubKeyTwo:
...

Load the file(successful), std::cout << (file appears proper), then try
for(auto it=node["items"].begin();it!=node["items"].end();++it)
{
   std::cout << it->first << std::endl;
}

calls to it->first blow up [TypedBadConversion] as it->type evaluates as Null.  
This should result in a Scalar (string) of "Key".

If I backup one step and ask for 
std::cout << it
then I will see
Key:
   SubKeyOne: [] #sequence of integers, in the order they are dependent
   SubKeyTwo:
which is correct!
Am I applying the iterator followed by ->first and ->second technique found in 
the Tutorial page incorrectly?

Original issue reported on code.google.com by JohnMich...@gmail.com on 11 Feb 2013 at 4:01

[GoogleCodeExporter]

I should add that skipping the cout overloads and directly trying
std::string temp = it->first.as<std::string>();
also throws this TypedBadConversion

Original comment by JohnMich...@gmail.com on 11 Feb 2013 at 4:04

[GoogleCodeExporter]

In your YAML file, node["items"] is a sequence, so when you iterate through, 
the iterators point to single entries (the entries of the sequence), not 
key/value pairs.

In other words:

for(auto it=node["items"].begin();it!=node["items"].end();++it) {
   YAML::Node entry = *it;
   // now, you can either iterate through the map:
   for(auto mapIt=entry.begin();mapIt!=entry.end();++mapIt) {
      std::cout << mapIt->first << "\n";
   }
   // or you can access by key:
   std::cout << entry["Key"] << "\n";
}

I will interpret this issue as a request for better a error message when you 
use the wrong dereferencing on an iterator :)

Original comment by jbe...@gmail.com on 11 Feb 2013 at 4:13

• Changed title: Better error messages when dereferencing (e.g.) a seq iterator as if it were a map iterator
• Changed state: Accepted
• Added labels: Component-New-API

[GoogleCodeExporter]

OK.  This seems counter-intuitive to me as both types of collections are 
defined as being composed of Nodes 
(http://www.yaml.org/spec/1.2/spec.html#id2764044).  Is there an implementation 
reason why the iterators can't act like Nodes by default?

Original comment by JohnMich...@gmail.com on 11 Feb 2013 at 4:29

[GoogleCodeExporter]

Ok, after reading the specification very carefully I believe I see the issue.  
Mapping nodes are defined as an unordered set of key/value pairs of any size.  
Hence single key/value nodes are not immediately distinguishable from larger 
maps.

I would propose a convenience method, although I can understand if one might 
have style objections.
Define first and second for YAML::Node.  Those methods would look like

YAML::Node first()
if(node.Type() != NodeType::Map)
  throw NodeNotMap
else
  return keys[0];

YAML::Node second()
if(node.Type() != NodeType::Map)
  throw NodeNotMap
else
  return values[0];

In the case of single key/value nodes the return values are intuitive.  In the 
case of maps > 1 the function would return the first key or value stored in 
memory.  This is not guaranteed to be ordered (and hence it is really only 
useful for maps with 1 k/v pair), but this keeps it fast and this would be 
noted in the documentation.  One example use case of this would be putting maps 
inside a sequence in order to create an ordered-map.  This method of creating 
ordered-maps is 1.2 compliant and thus isn't dependent on the parser.  
Offtopic: it's enhanced in appearance by the map-in-sequence shortcut: 
http://www.yaml.org/YAML_for_ruby.html#mapping-in-sequence_shortcut

Original comment by JohnMich...@gmail.com on 11 Feb 2013 at 5:11

[GoogleCodeExporter]

Yeah, I've thought about what an interface to access an ordered map would look 
like, but I'm not sure if this is the right thing to do. I'm not a huge fan of 
adding first() and second() methods on Node.

I will keep in mind the general question, thought. Thanks for the suggestion!

Original comment by jbe...@gmail.com on 11 Feb 2013 at 6:29

[GoogleCodeExporter]

Fixed, 
https://code.google.com/p/yaml-cpp/source/detail?r=09685b5f18dd422ed04c6cdd53d9e
931585491b8.

Original comment by jbe...@gmail.com on 13 Apr 2013 at 4:43

• Changed state: Fixed