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h1. Exceptions

GammaLib implements error handling using exceptions. The exceptions are implemented by the @GException@ class. GammaLib distinguishes between *logic exceptions* and *runtime exceptions*.

h2. Logic exceptions

Logic exceptions occur in situations that a client could have tested. They comprise:
* @invalid_value@: a value is invalid
* @invalid_argument@: an argument passed to a method or a function is invalid
* @out_of_range@: a value is outside its valid range
* @fits_error@: an error occurred in a cfitsio routine

h2. Runtime exceptions

Runtime exceptions occur in situations that a client can not test. They comprise:
* @underflow_error@
* @overflow_error@
* @feature_not_implemented@: the requested feature is not yet implemented in GammaLib

h2. Example code

Below an example code that illustrates how exceptions should be implemented:
<pre>
    if (num != m_cube.nmaps() ) {
        std::string msg = "Number of energies in 'ENERGIES' extension"
                          " ("+gammalib::str(num)+") does not match the"
                          " number of maps ("+gammalib::str(m_cube.nmaps())+""
                          " in the map cube.\n"
                          "The 'ENERGIES' extension table shall provide"
                          " one enegy value for each map in the cube.";
        throw GException::invalid_value(G_LOAD, msg);
    }
</pre>
An error message should be composed using a @std::string@. This message is then passed to @invalid_value@, @invalid_argument@ or any of the other standard exceptions. The @G_LOAD@ macro defines the name of the method that actually throws the exception (and is defined in the header of the @.cpp@ file):
<pre>
#define G_LOAD                 "GModelSpatialDiffuseCube::load(std::string&)"
</pre>
Note that arguments are given in this definition without the @const@ declaration and without the parameter name. If more than a single parameter exists, the parameter should be separated by a blank character:
<pre>
#define G_MC          "GModelSpatialDiffuseCube::mc(GEnergy&, GTime&, GRan&)"
</pre>