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boost::proto::functional::has_env_var — A unary boolean PolymorphicFunctionObject used for determining whether a particular transform environment has a value associated with a particular key.
// In header: <boost/proto/transform/env.hpp> template<typename Key> struct has_env_var : proto::callable { // member classes/structs/unions template<typename Sig> struct result { // types typedefsee-belowtype; }; // public member functions template<typename Env>see-belowoperator()(Env const &) const; };
has_env_var public member functionstemplate<typename Env> see-below operator()(Env const & e) const;
This function behaves as follows:
proto::is_env<Env>::value is true:
                      e[Key()] returns an instance of
                          proto::key_not_found, return
                          mpl::false_. See proto::env::operator[]
                          for more information.
                        mpl::true_.
                        Key is proto::data_type,
                          return mpl::true_.
                        mpl::false_.