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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.


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

struct deduce_domain {


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 and all its sub-domains are weaker than all other domains.
  • For each child, define a set of domains SN that includes the child's domain and all its super-domains.
  • Define a set IS that is the intersection of all the individual sets SN that don't contain proto::default_domain.
  • Define a set IW that is the intersection of all the individual sets SN that contain proto::default_domain.
  • Define a set P that is the union of IS and IW.
  • The common domain is the strongest domain in set P, with the following caveat.
  • Let U be the union of all sets SN. If the result is proto::default_domain and U contains an element that is not proto::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.