Under the hood

Registry classes

Registry classes statically hold instances of derived classes, and can be used to allocate a derived class dependent on some text string. The code thread below illustrates how the GModels::read() method extracts models from an XML file, allocates the requested model through the GModelRegistry::alloc() method, and appends the model to the model container.

C++

int n = lib->elements("source");                           // Get number of sources
for (int i = 0; i < n; ++i) {                              // Loop over sources
    const GXmlElement* src = lib->element("source", i);    // Get pointer on source
    std::string type = src->attribute("type");             // Get source model type
    GModelRegistry registry;                               // Get model registry
    GModel*        ptr = registry.alloc(type);             // Allocate model of specific type
    if (ptr != NULL) {                                     // If model is valid:
        ptr->read(*src);                                   // - read model
        append(*ptr);                                      // - append it to container
        delete ptr;                                        // - free model
    }
}

To access the registry information it is sufficient to create an instance of the registry. To illustrate how the registry is filled the src/model/GModelSky.cpp file can be examined:

C++

const GModelSky         g_pointsource_seed("PointSource");
const GModelSky         g_extendedsource_seed("ExtendedSource");
const GModelSky         g_diffusesource_seed("DiffuseSource");
const GModelSky         g_compositesource_seed("CompositeSource");
const GModelRegistry    g_pointsource_registry(&g_pointsource_seed);
const GModelRegistry    g_extendedsource_registry(&g_extendedsource_seed);
const GModelRegistry    g_diffusesource_registry(&g_diffusesource_seed);
const GModelRegistry    g_compositesource_registry(&g_compositesource_seed);

Here four instances of GModelSky are created as global variables which are appended to the GModelRegistry registry by creating an instance of the registry which takes the address of the GModelSky instance as argument.

Registries are widely used throughout GammaLib for the handling of models and its components, and the handling of the observations of the different supported instruments.