U3D
Open-source, cross-platform 2D and 3D game engine built in C++
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Serialization

Classes that derive from Serializable can perform automatic serialization to binary or XML format by defining attributes. Attributes are stored to the Context per class. Scene load/save and network replication are both implemented by having the Node and Component classes derive from Serializable.

The supported attribute types are all those supported by Variant, excluding pointers and custom values.

Attributes can either refer to a member of the object or define setter & getter functions. Member attributes can also have post-set action: member function without arguments that is called every time when value is assigned to the attribute.

Zero-based enumerations are also supported, so that the enum values can be stored as text into XML files instead of just numbers.

The folowing macros can be used to define an attribute:

  • URHO3D_ATTRIBUTE: Member of the object. Shall be convertible from and to specified type.
  • URHO3D_ATTRIBUTE_EX: Member of the object. Post-set member function callback is called when attribute is set.
  • URHO3D_ACCESSOR_ATTRIBUTE: Getter and setter member functions. Attribute value of specified type is passed into setter and expected to be returned from getter.
  • URHO3D_CUSTOM_ATTRIBUTE: Getter and setter functional objects that are directly working with Variant value. See MakeVariantAttributeAccessor()
  • URHO3D_ENUM_ATTRIBUTE: 32-bit integer zero-based enumeration with human-readable names.
  • URHO3D_ENUM_ATTRIBUTE_EX: The same as URHO3D_ATTRIBUTE_EX, used for enumerations.
  • URHO3D_ENUM_ACCESSOR_ATTRIBUTE: The same as URHO3D_ACCESSOR_ATTRIBUTE, used for enumerations.
  • URHO3D_CUSTOM_ENUM_ATTRIBUTE: The same as URHO3D_CUSTOM_ATTRIBUTE, used for enumerations.

To implement side effects to attributes, the default attribute access functions in Serializable can be overridden. See OnSetAttribute() and OnGetAttribute().

Each attribute can have a combination of the following flags:

  • AM_FILE: Is used for file serialization (load/save.)
  • AM_NET: Is used for network replication.
  • AM_LATESTDATA: Frequently changing data for network replication, where only the latest values matter. Used for motion and animation.
  • AM_NOEDIT: Is an internal attribute and is not to be shown for editing.
  • AM_NODEID: Is a node ID and may need rewriting when instantiating scene content.
  • AM_COMPONENTID: Is a component ID and may need rewriting when instantiating scene content.

The default flags are AM_FILE and AM_NET. Note that it is legal to define neither AM_FILE or AM_NET, meaning the attribute has only run-time significance (perhaps for editing.)

See the existing engine classes e.g. in the Scene or Graphics subdirectories for examples on registering attributes using the URHO3D_ATTRIBUTE family of helper macros.