How to load parent and child entities from XML using tinyxml2?

Question

I'm using XML to store my level files for a game engine that I'm working on. I recently added entity parenting, where each entity has a vector of pointers to it's children, as well as a pointer to it's parent. Each entity has a method that takes a pointer to add a parent, which also adds it to the parent's vector of children. There is also a method that adds a child, which does the same thing but in reverse. Problem: I have no idea how to load entities with children from the XML. Here's what I want a typical scene file to look like (I already have code to load entities and their components, I'm just not sure how to load parenting)

<scene>
    <entity name="Parent Entity">
        <transform posx="500" posy="100" scalex="1" scaley="1" rotation="0"/>
        <sprite image="Assets/Sprites/Avatar2.png"/>

        <entity name="Child Entity">
            <transform posx="0" posy="0" scalex="1" scaley="1" rotation="0"/>
            <sprite image="crimson-logo.png"/>
        </entity>
    </entity>

</scene>

So essentially what I need to do is loop over all the entities and create them using my entity manager's createEntity method (which returns a pointer to the new entity), check if they have children in the XML, and then if they do then call the parent entity's addChild method with a pointer to the child, or the current entity's addParent method with a pointer to the parent entity. I'm guessing I need to use a recursive function, but how would I write such a function?

Answer 1

This is a classical type of task where you use recursion. Recursion can be tricky as it is, but especially so when dealing with tinyxml - not the friendliest API.

STEP 1: A find Helper

Let's make it friendlier. At all levels we will want to visit all "entity" elements. Let's make a handy helper to use TinyXML to get those:

auto find(TiXmlElement const* node, char const* name) {
    std::vector<TiXmlElement const*> found;
    for (
            auto el = node->FirstChildElement(name);
            el;
            el = el->NextSiblingElement(name)
        )
    {
        found.push_back(el);
    }
    return found;
}

So far so good, now we can more easily query nodes with a certain name.

STEP 2: Parse a Single Entity

Let's forget about that helper for a second, and assume we already have an "entity" node selected. Now we want to parse it into an Entity (name, sprite, etc.) and return a newly created entity:

Entity* parse_entity(TiXmlElement const* node) {
    Entity* entity = g_manager.createEntity(node->Attribute("name"));
    // todo transforms, sprite info etc.
    return entity;
}

STEP 3: Build The Tree

That sounds like the complicated part. But really, using our find helper from above, all we need is this:

void parse_sub_entities(TiXmlElement const* node, Entity* parent = nullptr) {
    for (auto el : find(node, "entity")) {
        auto entity = parse_entity(el);

        if (parent && entity) {
            entity->addParent(parent);
            parent->addChild(entity);
        }

        parse_sub_entities(el, entity);
    }
}

All child nodes are parsed using parse_entity from Step #2, and only if we had a parent node we add the relationships.

To finish it all up, we recurse into child entities, this time passing the current entity as the parent.

Full Demo

**Not Live On Coliru**¹

#include <tinyxml.h>
#include <vector>
#include <list>
#include <iostream>
#include <iomanip>

namespace { // helper functions for XML searching
    auto find(TiXmlElement const* node, char const* name) {
        std::vector<TiXmlElement const*> found;
        for (
                auto el = node->FirstChildElement(name);
                el;
                el = el->NextSiblingElement(name)
            )
        {
            found.push_back(el);
        }
        return found;
    }
}

auto scene = R"(<scene>
    <entity name="Parent Entity">
        <transform posx="500" posy="100" scalex="1" scaley="1" rotation="0"/>
        <sprite image="Assets/Sprites/Avatar2.png"/>

        <entity name="Child Entity">
            <transform posx="0" posy="0" scalex="1" scaley="1" rotation="0"/>
            <sprite image="crimson-logo.png"/>
        </entity>
    </entity>

</scene>)";

struct Entity {
    std::string _name;
    Entity* _parent = nullptr;
    std::vector<Entity*> _children;

    explicit Entity(std::string name = "unnamed") : _name(std::move(name)) {}

    void addChild(Entity* e) { _children.push_back(e); }
    void addParent(Entity* e) {
        assert(!_parent || _parent == e);
        _parent = e;
    }
};
struct Mgr {
    std::list<Entity> _entities;
    Entity* createEntity(std::string name) {
        return &_entities.emplace_back(name);
    };
} g_manager;

Entity* parse_entity(TiXmlElement const* node) {
    Entity* entity = g_manager.createEntity(node->Attribute("name"));
    // todo transforms, sprite info etc.
    return entity;
}

void parse_sub_entities(TiXmlElement const* node, Entity* parent = nullptr) {
    for (auto el : find(node, "entity")) {
        auto entity = parse_entity(el);

        if (parent && entity) {
            entity->addParent(parent);
            parent->addChild(entity);
        }

        parse_sub_entities(el, entity);
    }
}

int main() {
    TiXmlDocument doc;
    doc.Parse(scene);

    parse_sub_entities(doc.RootElement());

    std::cout << "Manager has " << g_manager._entities.size() << " entities\n";

    for (auto& e: g_manager._entities) {
        std::cout << "==== Entity: " << std::quoted(e._name) << "\n";
        if (e._parent)
            std::cout << " - has parent " << std::quoted(e._parent->_name) << "\n";
        for (auto child : e._children)
            std::cout << " - has child " << std::quoted(child->_name) << "\n";
    }
}

Prints:

Manager has 2 entities
==== Entity: "Parent Entity"
 - has child "Child Entity"
==== Entity: "Child Entity"
 - has parent "Parent Entity"

¹ no tinyxml installed on any online compiler I know