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Contents
- Basic Parsing
- Reading From the Document
- Getting More Complicated
- A Complete Example
- When Something Goes Wrong
- Note about copying YAML::Node
Basic Parsing
The parser accepts streams, not file names, so you need to first load the file. Since a YAML file can contain many documents, you can grab them one-by-one. A simple way to parse a YAML file might be:
#include <fstream> #include "yaml-cpp/yaml.h" int main() { std::ifstream fin("test.yaml"); YAML::Parser parser(fin); YAML::Node doc; while(parser.GetNextDocument(doc)) { // ... } return 0; }
Reading From the Document
Suppose we have a document consisting only of a scalar. We can read that scalar like this:
YAML::Node doc; // let's say we've already parsed this document std::string scalar; doc >> scalar; std::cout << "That scalar was: " << scalar << std::endl;
How about sequences? Let's say our document now consists only of a sequences of scalars. We can use an iterator:
YAML::Node doc; // already parsed for(YAML::Iterator it=doc.begin();it!=doc.end();++it) { std::string scalar; *it >> scalar; std::cout << "Found scalar: " << scalar << std::endl; }
... or we can just loop through:
YAML::Node doc; // already parsed for(unsigned i=0;i<doc.size();i++) { std::string scalar; doc[i] >> scalar; std::cout << "Found scalar: " << scalar << std::endl; }
And finally maps. For now, let's say our document is a map with all keys/values being scalars. Again, we can iterate:
YAML::Node doc; // already parsed for(YAML::Iterator it=doc.begin();it!=doc.end();++it) { std::string key, value; it.first() >> key; it.second() >> value; std::cout << "Key: " << key << ", value: " << value << std::endl; }
Note that dereferencing a map iterator is undefined; instead, use the first and second methods to get the key and value nodes, respectively.
Alternatively, we can pick off the values one-by-one, if we know the keys:
YAML::Node doc; // already parsed std::string name; doc["name"] >> name; int age; doc["age"] >> age; std::cout << "Found entry with name '" << name << "' and age '" << age << "'\n";
One thing to be keep in mind: reading a map by key (as immediately above) requires looping through all entries until we find the right key, which is an O(n) operation. So if you're reading the entire map this way, it'll be O(n^2). For small n, this isn't a big deal, but I wouldn't recommend reading maps with a very large number of entries (>100, say) this way.
Optional Keys
If you try to access a key that doesn't exist, yaml-cpp throws an exception (see HowToParseADocument#When_Something_Goes_Wrong When Something Goes Wrong). If you have optional keys, you it's often easier to use FindValue instead of operator[]:
YAML::Node doc; // already parsed if(const YAML::Node *pName = doc.FindValue("name")) { std::string name; *pName >> name; std::cout << "Key 'name' exists, with value '" << name << "'\n"; } else { std::cout << "Key 'name' doesn't exist\n"; }
Getting More Complicated
The above three methods can be combined to read from an arbitrary document. But we can make life a lot easier. Suppose we're reading 3-vectors (i.e., vectors with three components), so we've got a structure looking like this:
struct Vec3 { float x, y, z; };
We can read this in one operation by overloading the extraction (>>) operator:
void operator >> (const YAML::Node& node, Vec3& v) { node[0] >> v.x; node[1] >> v.y; node[2] >> v.z; } // now it's a piece of cake to read it YAML::Node doc; // already parsed Vec3 v; doc >> v; std::cout << "Here's the vector: (" << v.x << ", " << v.y << ", " << v.z << ")\n";
A Complete Example
Here's a complete example of how to parse a complex YAML file:
monsters.yaml
- name: Ogre position: [0, 5, 0] powers: - name: Club damage: 10 - name: Fist damage: 8 - name: Dragon position: [1, 0, 10] powers: - name: Fire Breath damage: 25 - name: Claws damage: 15 - name: Wizard position: [5, -3, 0] powers: - name: Acid Rain damage: 50 - name: Staff damage: 3
main.cpp
#include "yaml-cpp/yaml.h" #include <iostream> #include <fstream> #include <string> #include <vector> // our data types struct Vec3 { float x, y, z; }; struct Power { std::string name; int damage; }; struct Monster { std::string name; Vec3 position; std::vector <Power> powers; }; // now the extraction operators for these types void operator >> (const YAML::Node& node, Vec3& v) { node[0] >> v.x; node[1] >> v.y; node[2] >> v.z; } void operator >> (const YAML::Node& node, Power& power) { node["name"] >> power.name; node["damage"] >> power.damage; } void operator >> (const YAML::Node& node, Monster& monster) { node["name"] >> monster.name; node["position"] >> monster.position; const YAML::Node& powers = node["powers"]; for(unsigned i=0;i<powers.size();i++) { Power power; powers[i] >> power; monster.powers.push_back(power); } } int main() { std::ifstream fin("monsters.yaml"); YAML::Parser parser(fin); YAML::Node doc; parser.GetNextDocument(doc); for(unsigned i=0;i<doc.size();i++) { Monster monster; doc[i] >> monster; std::cout << monster.name << "\n"; } return 0; }
When Something Goes Wrong
... we throw an exception (all exceptions are derived from YAML::Exception). If there's a parsing exception (i.e., a malformed YAML document), we throw a YAML::ParserException:
try { std::ifstream fin("test.yaml"); YAML::Parser parser(fin); YAML::Node doc; parser.GetNextDocument(doc); // do stuff } catch(YAML::ParserException& e) { std::cout << e.what() << "\n"; }
If you make a programming error (say, trying to read a scalar from a sequence node, or grabbing a key that doesn't exist), we throw some kind of YAML::RepresentationException. To prevent this, you can check what kind of node something is:
YAML::Node node; YAML::NodeType::value type = node.Type(); // should be: // YAML::NodeType::Null // YAML::NodeType::Scalar // YAML::NodeType::Sequence // YAML::NodeType::Map
Note about copying YAML::Node
Currently YAML::Node is non-copyable, so you need to do something like
const YAML::Node& node = doc["whatever"];
This is intended behavior. If you want to copy a node, use the Clone function:
std::auto_ptr<YAML::Node> pCopy = myOtherNode.Clone();
The intent is that if you'd like to keep a YAML::Node around for longer than the document will stay in scope, you can clone it and store it as long as you like.
Thanks , Jørn! However:
1. This doesn't appear to provide any extra functionality during the traversal (e.g., no callbacks) - it just dumps the node, and yaml-cpp already has that functionality (the emitter can emit Yaml::Nodes)
2. It doesn't provide any mechanism for quoting strings with nonprintable or newline characters.
3. It only handles maps with strings as keys (so it doesn't recurse to keys).
So I don't think I'll add anything like this (at least for the time being). Thanks for your interest!
Hello, just wanted to contribute a recursive depth first function, for a yaml-cpp YAML::Node document:
Tested with g++ (GCC) 3.2.2.
Thanks, Jørn.