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Actors in Detail


The main concept is the Actor. An Actor is a model of the Polymorphic Function Object concept (that can accept 0 to N arguments (where N is a predefined maximum).

An Actor contains a valid Phoenix Expression, a call to one of the function call operator overloads, starts the evaluation process.

[Note] Note

You can set BOOST_PHOENIX_LIMIT, the predefined maximum arity an actor can take. By default, BOOST_PHOENIX_LIMIT is set to 10.

The actor template class models the Actor concept:

template <typename Expr>
struct actor
    template <typename Sig>
    struct result;

    typename result_of::actor<Expr>::type
    operator()() const;

    template <typename T0>
    typename result_of::actor<Expr, T0 &>::type
    operator()(T0& _0) const;

    template <typename T0>
    typename result_of::actor<Expr, T0 const &>::type
    operator()(T0 const & _0) const;


Table 1.9. Actor Concept Requirements



actor(arg0, arg1, ..., argN)

Function call operators to start the evaluation

boost::result_of<Actor<Expr>(Arg0, Arg1, ..., ArgN)>::type

Result of the evaluation

result_of::actor<Expr, Arg0, Arg1, ..., ArgN>::type

Result of the evaluation

Function Call Operators

There are 2*N function call operators for 0 to N arguments (N == BOOST_PHOENIX_LIMIT). The actor class accepts the arguments and forwards the arguments to the default evaluation action.

Additionally, there exist function call operators accepting permutations of const and non-const references. These operators are created for all N <= BOOST_PHOENIX_PERFECT_FORWARD_LIMIT (which defaults to 3).

[Note] Note

Forwarding Function Problem

There is a known issue with current C++ called the "Forwarding Function Problem". The problem is that given an arbitrary function F, using current C++ language rules, one cannot create a forwarding function FF that transparently assumes the arguments of F.


On an actor function call, before calling the evaluation function, the actor created a context. This context consists of an Environment and an Action part. These contain all information necessary to evaluate the given expression.

Table 1.10. Context Concept Requirements



result_of::context<Env, Actions>::type

Type of a Context

context(e, a)

A Context containing environment e and actions a


Type of the contained Environment


The environment


Type of the contained Actions


The actions


The Environment is a model of Random Access Sequence.

The arguments passed to the actor's function call operator are collected inside the Environment:

Other parts of the library (e.g. the scope module) extends the Environment concept to hold other information such as local variables, etc.


Actions is the part of Phoenix which are responsible for giving the actual expressions a specific behaviour. During the traversal of the Phoenix Expression Tree these actions are called whenever a specified rule in the grammar matches.

struct actions
    template <typename Rule>
    struct when;

The nested when template is required to be Proto Primitive Transform. No worries, you don't have to learn Boost.Proto just yet! Phoenix provides some wrappers to let you define simple actions without the need to dive deep into proto.

Phoenix ships with a predefined default_actions class that evaluates the expressions with C++ semantics:

struct default_actions
    template <typename Rule, typename Dummy = void>
    struct when
        : proto::_default<meta_grammar>

For more information on how to use the default_actions class and how to attach custom actions to the evaluation process, see more on actions.

struct evaluator
    template <typename Expr, typename Context>
    unspecified operator()(Expr &, Context &);

evaluator const eval = {};

The evaluation of a Phoenix expression is started by a call to the function call operator of evaluator.

The evaluator is called by the actor function operator overloads after the context is built up. For reference, here is a typical actor::operator() that accepts two arguments:

template <typename T0, typename T1>
typename result_of::actor<Expr, T0 &, T1 &>::type
operator()(T0 &t0, T1 &t1) const
    fusion::vector2<T0 &, T1 &> env(t0, t1);

    return eval(*this, context(env, default_actions()));

For reasons of symmetry to the family of actor::operator() there is a special metafunction usable for actor result type calculation named result_of::actor. This metafunction allows us to directly specify the types of the parameters to be passed to the actor::operator() function. Here's a typical actor_result that accepts two arguments:

namespace result_of
    template <typename Expr, typename T0, typename T1>
    struct actor
        typedef fusion::vector2<T0, T1>                                           env_tpe;
        typedef typename result_of::context<env_type, default_actions>::type      ctx_type
        typedef typename boost::result_of<evaluator(Expr const&, ctx_type)>::type type;