This is fancy object-oriented speak for allowing you to define a set of methods which can be implemented by any class which can then be queried for later. The main advantage of modeling overridable methods with this approach is that it doesn't require a type to inherit or derive from a common subtype. Any class inheriting from any other can implement any number of interfaces and inherit their implementations from parent classes.

LibGimbal interfaces support:

abstract overridable methods
overridable methods with a default implementation
static members
implementing other interfaces
inheriting from other interfaces

Declaring

All interfaces in libGimbal derive from the base type, GBL_INTERFACE_TYPE. This is a class-only, abstract type which defines the base class which we will use: GblInterface.

Structures

We can create our own interface class sructure by deriving from GblInterface:

// lets create an interface to implement saving/loading
GBL_INTERFACE_DERIVE(ISerializable)
    // declare virtual methods for implementing save/load
    GBL_RESULT  (*pFnSave)(const ISerializable* pSelf, GblStringBuffer* pBuffer);
    GBL_RESULT  (*pFnLoad)(ISerializable* pSelf, const GblStringBuffer* pBuffer);
GBL_INTERFACE_END

Note: It is not a hard requirement that interface methods must return a GBL_RESULT type; however, this can be extremely convenient for propagating errors should one occur within the implementation. Since these methods are not what are typically called by a user, it doesn't make your API any uglier to do so (see the next section).

Macro Utilities

As is typical with most libGimbal types, it is often most convenient on the user (and you) to define a set of common macro operators for working with your type.

For our serializable interface, we will use the following:

// convenience macro returning type ID from registration
#define ISERIALIZABLE_TYPE                (GBL_TYPEID(ISerializable))
 
// function-style cast operator from an generic instance to our instance type
#define ISERIALIZABLE(instance)           (GBL_CAST(instance, ISerializable))
 
// function-style cast operator from a generic class to our class/interface type
#define ISERIALIZABLE_CLASS(klass)        (GBL_CLASS_CAST(klass, ISerializable))
 
// convenience macro to extract our interface from a generic instance
#define ISERIALIZABLE_GET_CLASS(instance) (GBL_CLASSOF(instance, ISerializable))

Public Methods

Typically, when working with interface methods, we would rather provide a user-friendly API function wrapping the virtual method and handling any errors, rather than making a user reach into an interface and call a function pointer directly.

We do this for our virtual save method:

GBL_RESULT ISerializable_save(const ISerializable* pSelf, GblStringBuffer* pBuffer) {
    GBL_CTX_BEGIN(NULL);
 
    // exttract our interface from the given instance
    ISerializableClass* pClass = ISERIALIZABLE_GET_CLASS(pSelf);
 
    // check whether we managed to find the interface
    if(pClass) {
 
        // check whether the virtual method has been implemented
        if(pClass->pFnSave) {
 
            // call virtual method implementation,
            // propagating any error code returned
            GBL_CTX_VERIFY_CALL(pClass->pFnSave(pSelf, pBuffer);
        }
    }
    GBL_CTX_END();
}

As you can see, when we expose our virtual methods via a public API wrapper, the entry-point becomes prettier than calling directly into a function pointer, and we are able to do type checking and error handling. We can check to see whether the given instance was even compatible with our interface or whether the class implementing the interface actually overrode the method or not.

Registering

Once we've defined our structures, created our utility macros, and created a public API around our virtual methods, it's time to register our type. To do this, we implement the ISerializable_type() function declared earlier to register a new meta type if we haven't already:

GblType ISerializable_type(void) {
    // declare static variable, so we store the value
    static GblType type = GBL_INVALID_TYPE;
 
    // only register if this is our first time getting called
    if(type == GBL_INVALID_TYPE) {
 
        // register our interface's class, deriving from GBL_INTERFACE_TYPE
        type = GblType_register("ISerializable",
                                      GBL_INTERFACE_TYPE,
                                      &(const GblTypeInfo) {
                                          .classSize = sizeof(ISerializableClass)
                                      },
                                      GBL_TYPE_FLAG_ABSTRACT);
    }
 
    return type;
}

Note: If we wish to provide a default implementation of our virtual methods, we would also set GblTypeInfo::pFnClassInit to a GblClassInitFn function where we would initialize our class structure with some default values.

Implementing

In order to use your interface with a given type, the type must implement the interface and then provide the meta type system with a mapping during type registration.

Structures

If we wish to implement our interface on another type, we embed it within that type's class structure. Here we will use the libGimbal macro DSL which will handle generating our structures for us.

// Class structure (base class with one interface being implemented)
GBL_CLASS_BASE(IntSerializable, ISerializable)
// any extra static variables or virtual functions here
GBL_CLASS_END
 
// could've used GBL_CLASS_BASE_EMPTY() if we had nothing extra to add
 
// Instance structure
GBL_INSTANCE_BASE(IntSerializable)
    int integer; //single instance member
GBL_INSTANCE_END

Overrides

Finally, lets create an implementation of the save and load functions from the ISerializableIFace class, along with a class constructor for initializing IntSerializableClass:

// IntSerializable's implementation of ISerializable_load()
GBL_RESULT IntSerializable_load_(ISerializable* pSelf, const GblStringBuffer* pBuffer) {
    IntSerializable* pInt = (IntSerializable*)pSelf;
 
    // load an integer from the string buffer
    pInt->integer = GblStringView_toInt(GblStringBuffer_view(pBuffer));
 
}
 
// IntSerializable's implementation of ISerialiable_save();
GBL_RESULT IntSerializable_save_(const ISerializable* pSelf, GblStringBuffer* pBuffer) {
    IntSerializable* pInt = (IntSerializable*)pSelf;
 
    // save the integer to the string buffer
    return GblStringBuffer_appendInt(pBuffer, pInt->integer);
}
 
// IntSerializable's class initializer, called when the class is created
GBL_RESULT IntSerializable_initializeClass(GblClass* pClass, const void* pUd, GblContext* pCtx) {
    IntSerializableClass* pSelfClass = (IntSerializableClass*)pClass;
 
    // override the interface implementation for the class
    pSelfClass->iSerializable.pFnLoad = IntSerializable_load_;
    pSelfClass->iSerializable.pFnSave = IntSerializable_save_;
}

Registration

In order to register a type as having implemented an interface, we have to tell the meta type system how to "map" between the interface and the class. In order to achieve this, we pass an array of interface mappings to GblType_register() via GblTypeInfo.pInterfaceImpls:

GblType IntSerializableType_type(void) {
    static GblType type = GBL_INVALID_TYPE;
 
    if(type == GBL_INVALID_TYPE) {
 
        type = GblType_register("IntSerializable",
                               GBL_INSTANCE_TYPE,
                               &(const GblTypeInfo) {
                               .classSize       = sizeof(IntSerializableClass),
                               .pFnClassInit    = IntSerializable_initializeClass,
                               .instanceSize    = sizeof(IntSerializable),
                               .interfaceCount  = 1,
                               .pInterfaceImpls   = (const GblInterfaceImpl[]) {
                                   {
                                       .interfaceType = ISERIALIZABLE_TYPE,
                                       .classOffset   = offsetof(IntSerializableClass, iSerializable)
                                   }
                           },
                           GBL_TYPE_FLAGS_NONE);
     }
     return type;
}

As you can see, we provided a single entry into the interface mapping, which we used to associate our interface type with the given class offset. Now the meta type system knows everything it needs to be able to cast to and from your interface!

Querying

Querying for interfaces is extremely simple. Since they're essentially a type of GblClass, you use the same set of functions you would use to cast between class types. Lets create an instance of the IntSerializable type and try to serialize it with our interface using the utility macros defined earlier to handle casting:

// create an instance
IntSerializable* pIntInstance = GblInstance_create(intSerializeType);
 
// create a new string buffer to hold the data
GblStringBuffer buffer;
GblStringBuffer_construct(&buffer, GBL_STRV("7"));
 
// cast our type to an ISerializable and call the virtual function
ISerializable_load(ISERIALIZABLE(pIntInstance), &buffer);
 
// look, we loaded the initial value of 7!
assert(pIntInstance->integer == 7);
 
pIntInstance->integer = -7;
 
// we can now save a value back too!
GblStringBuffer_clear(&buffer);
ISerializable_save(ISERIALIZABLE(pIntInstance), &buffer));
 
// look, it saved the new value!
assert(GblStringView_toInt(GblStringBuffer_view(&buffer)) == -7);
 
// destroy out instance when we're done
GblInstance_destroy(pIntInstance);

Querying for the GblInterface structure without having defined the convenience macros from the previous section is still possible, but it's much uglier and more verbose.

// A. Cast to an ISerializable instance from IntSerializable
ISerializable* pSerializable = (ISerializable*)GblInstance_cast((GblInstance*)pIntInstance,
                                                                GBL_ISERIALIZABLE_TYPE);
 
// B. Extract the ISerializableIFace class from IntSerializable
 
//    1. retrieve the class from the instance
GblClass* pClass = GblInstance_class((GblInstance*)pSerializable);
 
//    2. retrieve the interface from the class
ISerializableIFace* pIFace = (ISerializableIFace*)GblClass_cast(pClass, GBL_ISERIALIZABLE_TYPE);

Table of Contents

Declaring

Structures

Macro Utilities

Public Methods

Registering

Implementing

Structures

Overrides

Registration

Querying