Struct deduce

Struct deduce_domain

boost::proto::deduce_domain — A pseudo-domain for use in functions and metafunctions that require a domain parameter. It indicates that the domain of the parent node should be inferred from the domains of the child nodes.

Synopsis

// In header: <boost/proto/domain.hpp>


struct deduce_domain {
};

Description

When proto::deduce_domain is used as a domain — either explicitly or implicitly by proto::make_expr(), proto::unpack_expr(), or Proto's operator overloads — Proto will use the domains of the child expressions to compute the domain of the parent. It is done in such a way that (A) expressions in domains that share a common super-domain are interoperable, and (B) expressions that are in the default domain (or a sub-domain thereof) are interoperable with all expressions. The rules are as follows:

A sub-domain is stronger than its super-domain.
proto::default_domain, proto::basic_default_domain and all their sub-domains are weaker than all other domains.
proto::basic_default_domain is weaker than proto::default_domain.
For each child, define a set of domains S_N that includes the child's domain and all its super-domains.
Define a set I_S that is the intersection of all the individual sets S_N that don't contain proto::default_domain or proto::basic_default_domain.
Define a set I_W that is the intersection of all the individual sets S_N that contain proto::default_domain or proto::basic_default_domain.
Define a set P that is the union of I_S and I_W.
The common domain is the strongest domain in set P, with the following caveats.
Let U be the union of all sets S_N. If the result is proto::default_domain or proto::basic_default_domain and U contains an element that is not proto::default_domain or proto::basic_default_domain, it is an error.

Note: the above description sounds like it would be expensive to compute at compile time. In fact, it can all be done using C++ function overloading.