boost::openmethod::method

Declared in <boost/openmethod/core.hpp>

Methods are created by specializing the method class template with an identifier, a function type and optionally a registry.

Id is a type, typically an incomplete class declaration named after the method's purpose. It is used to allow different methods with the same signature.

Fn is a function type, i.e. a type in the form ReturnType(Parameters...).

Registry is an instantiation of class template registry. Methods may use only classes that have been registered in the same registry as virtual parameters and arguments. The registry also contains a set of policies that influence several aspects of the dispatch mechanism ‐ for example, how to acquire a v‐table pointer for an object, how to report errors, whether to perform sanity checks, etc.

The default value for Registry is default_registry, but it can be overridden by defining the preprocessor symbol BOOST_OPENMETHOD_DEFAULT_REGISTRY, before including <boost/openmethod/core.hpp>. Setting the symbol afterwards has no effect.

Specializations of method have a single instance: the static member fn, which has an operator() that forwards to the appropriate overrider. It is selected in the same way as overloaded function resolution:

1. Form the set of all applicable overriders. An overrider is applicable if it can be called with the arguments passed to the method.

2. If the set is empty, call the error handler (if present in the registry), then terminate the program with abort.

3. Remove the overriders that are dominated by other overriders in the set. Overrider A dominates overrider B if at least one of its virtual formal parameters is more specialized than B's, and if none of B's virtual parameters is more specialized than A's.

4. If the resulting set contains exactly one overrider, call it.

If a single most specialized overrider does not exist, the program is terminated via abort. If the registry contains an error_handler policy, its error function is called with an object that describes the error, prior calling abort. error may prevent termination by throwing an exception.

For each virtual argument arg, the dispatch mechanism calls virtual_traits::peek(arg) and deduces the v‐table pointer from the result, using the first of the following methods that applies:

1. If result is a virtual_ptr, get the pointer to the v‐table from it.

2. If boost_openmethod_vptr can be called with result and a Registry*, and it returns a vptr_type, call it.

3. Call Registry::rtti::dynamic_vptr(result).

If Registry was initialized with the N2216 option, ambiguous calls are not an error. Instead, the following extra steps are taken to select an overrider:

1. If the return type is a registered polymorphic type, remove all the overriders that return a less specific type than others.

2. If the resulting set contains only one overrider, call it.

3. Otherwise, call one of the remaining overriders. Which overrider is selected is not specified, but it is the same across calls with the same arguments types.

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