Updated over 11 years ago by 
How to create a cast in Python?¶
Ideally, one would like to avoid casts, as they indicate a poorly defined interface. However, sometimes one still would like to have a cast. An example is the GInstDir class, which defines an abstract direction in the instrument system. Sometimes, the direction has nothing to do with a position on the sky (for example for the SPI telescope on INTEGRAL which is a non-imaging telescope), sometimes the instrument direction is identical to a sky position (for all imaging telescopes). So how can we get out of GInstDir object the sky direction? We need a cast.
A cast can be implement as a class extension in the SWIG interface. Here a cast for the GCTAInstDir class:
%extend GCTAInstDir {
GCTAInstDir(GInstDir* dir) {
GCTAInstDir* ptr = dynamic_cast<GCTAInstDir*>(dir);
if (ptr != NULL) {
return (ptr->clone());
}
else {
throw GException::bad_type("GCTAInstDir(GInstDir*)", "GInstDir not of type GCTAInstDir");
}
}
};
It is used for example as in
instdir = GCTAInstDir(dir)
Note that the extension does in fact not really implement a cast, but it provides a deep copy of the instrument direction. The method also verifies that the cast is valid.
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