CXXR (C++ R) API
Public Member Functions | Static Public Member Functions | Protected Member Functions | Friends
CXXR::Promise Class Reference

Mechanism for deferred evaluation. More...

#include <Promise.h>

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List of all members.

Public Member Functions

 Promise (RObject *valgen, Environment *env)
Environmentenvironment () const
 Access the environment of the Promise.
RObjectforce ()
 Force the Promise.
bool isMissingSymbol () const
 Not for general use.
void setValue (RObject *val)
 Set value of the Promise.
RObjectvalue ()
 Access the value of a Promise.
const RObjectvalueGenerator () const
 RObject to be evaluated by the Promise.
RObjectevaluate (Environment *env)
 Evaluate object in a specified Environment.
const char * typeName () const
 Name within R of this type of object.
void visitReferents (const_visitor *v) const
 Conduct a visitor to the nodes referred to by this one.
- Public Member Functions inherited from CXXR::RObject
virtual const PairListattributes () const
 Get object attributes.
virtual void clearAttributes ()
 Remove all attributes.
virtual RObjectclone () const
 Return pointer to a copy of this object.
void copyAttribute (const Symbol *name, const RObject *source)
 Copy an attribute from one RObject to another.
void copyAttributes (const RObject *source, bool copyS4)
 Copy attributes from one RObject to another.
virtual RObjectgetAttribute (const Symbol *name) const
 Get the value a particular attribute.
virtual bool hasAttributes () const
 Has this object any attributes?
bool hasClass () const
 Has this object the class attribute?
bool isS4Object () const
 Is this an S4 object?
void maybeTraceMemory (const RObject *src)
 Carry out memory tracing.
void maybeTraceMemory (const RObject *src1, const RObject *src2)
 Carry out memory tracing.
void maybeTraceMemory (const RObject *src1, const RObject *src2, const RObject *src3)
 Carry out memory tracing.
bool memoryTraced () const
 Is copying etc. of this object being traced?
virtual unsigned int packGPBits () const
 Reproduce the gp bits field used in CR.
virtual void setAttribute (const Symbol *name, RObject *value)
 Set or remove an attribute.
void setAttributes (const PairList *new_attributes)
 Replace the attributes of an object.
void setMemoryTracing (bool on)
 Enable/disable tracing of copying etc.
void setS4Object (bool on)
 Set the status of this RObject as an S4 object.
SEXPTYPE sexptype () const
 Get an object's SEXPTYPE.
virtual void unpackGPBits (unsigned int gpbits)
 Interpret the gp bits field used in CR.
- Public Member Functions inherited from CXXR::GCNode
void expose () const
 Record that construction of a node is complete.
bool isExposed () const
 Has this node been exposed to garbage collection?

Static Public Member Functions

static const char * staticTypeName ()
 The name by which this type is known in R.
- Static Public Member Functions inherited from CXXR::RObject
template<class T >
static T * clone (const T *pattern)
 Return a pointer to a copy of an object.
- Static Public Member Functions inherited from CXXR::GCNode
static void * operator new (size_t bytes)
 Allocate memory.
static void * operator new (size_t, void *where)
 Placement new for GCNode.
static void operator delete (void *p, size_t bytes)
 Deallocate memory.
static bool check ()
 Integrity check.
template<class T >
static T * expose (T *node)
 Record that construction of a node is complete.
static void gc ()
 Initiate a garbage collection.
static void gclite ()
 Lightweight garbage collection.
static void maybeCheckExposed (const GCNode *node)
 Subject to configuration, check that a GCNode is exposed.
static size_t numNodes ()
 Number of GCNode objects in existence.

Protected Member Functions

void detachReferents ()
 Null out all references from this node to other nodes.
- Protected Member Functions inherited from CXXR::RObject
 RObject (SEXPTYPE stype=CXXSXP)
 RObject (const RObject &pattern)
 Copy constructor.
- Protected Member Functions inherited from CXXR::GCNode
virtual ~GCNode ()

Friends

class boost::serialization::access

Additional Inherited Members

- Public Attributes inherited from CXXR::RObject
unsigned char m_named
unsigned m_missing: 2
unsigned m_argused: 2
bool m_active_binding: 1
bool m_binding_locked: 1

Detailed Description

Mechanism for deferred evaluation.

This class is used to handle function arguments within R's lazy evaluation scheme. A Promise object encapsulates a pointer to an arbitrary RObject (typically a Symbol or an Expression), and a pointer to an Environment. When the Promise is first evaluated, the RObject is evaluated within the Environment, and the result of evaluation returned as the value of the Promise.

After the first evaluation, the result of evaluation is cached within the Promise object, and the Environment pointer is set null (thus possibly allowing the Environment to be garbage-collected). Subsequent evaluations of the Promise object simply return the cached value.

Note:
When a Promise is evaluated via a call to evaluate() (the virtual function defined in class RObject), the env parameter to evaluate() is ignored: evaluation uses only the Environment encapsulated within the Promise object. When an RObject is known to be a Promise, it is suggested that evaluation be carried out using the function Promise::force(), which lacks the redundant parameter and is consequently clearer to readers of the code.

Constructor & Destructor Documentation

CXXR::Promise::Promise ( RObject valgen,
Environment env 
)
inline
Parameters:
valgenpointer to RObject to be evaluated to provide the value of the Promise. Can be null.
envpointer to the Environment in which valgen is to be evaluated. If this pointer is null, the value of the Promise is immediately set to be valgen itself.

Member Function Documentation

void CXXR::Promise::detachReferents ( )
protectedvirtual

Null out all references from this node to other nodes.

The referents of this node are those objects (derived from GCNode) designated by a GCEdge within this object. This function changes all GCEdges within this object to encapsulate a null pointer. It is used during the sweep phase of a mark-sweep garbage collection to break up unreachable subgraphs, and in particular to remove reference loops from them. After the application of this method, the GCNode should be regarded as a 'zombie', kept in existence only so other nodes can detach their references to it cleanly (using decRefCount()).

Note:
If this method is reimplemented in a derived class, the reimplemented version must remember to invoke detachReferents() for the immediate base class of the derived class, to ensure that all referents of the object get detached.

Reimplemented from CXXR::RObject.

Environment* CXXR::Promise::environment ( ) const
inline

Access the environment of the Promise.

Returns:
Pointer to the environment of the Promise. This will be a null pointer after the promise has been evaluated.
RObject* CXXR::Promise::evaluate ( Environment env)
virtual

Evaluate object in a specified Environment.

Parameters:
envPointer to the environment in which evaluation is to take place.
Returns:
Pointer to the result of evaluation.

Reimplemented from CXXR::RObject.

RObject* CXXR::Promise::force ( )
inline

Force the Promise.

i.e. evaluate the value generator of the Promise within the Environment of the Promise. Following this, the environment pointer is set null, thus possibly allowing the Environment to be garbage-collected.

If this function is used on a Promise that has already been forced, it simply returns the previously computed value.

Returns:
The value of the Promise, i.e. the result of evaluating the value generator.
bool CXXR::Promise::isMissingSymbol ( ) const

Not for general use.

This function is used by isMissingArgument(). It implements some logic from CR's R_isMissing() which I don't fully understand.

Returns:
true iff ... well, read the code!
void CXXR::Promise::setValue ( RObject val)

Set value of the Promise.

Once the value is set to something other than Symbol::unboundValue(), the environment pointer is set null.

Parameters:
valValue to be associated with the Promise.
Todo:
Should be private (or removed entirely), but currently still used in saveload.cpp.
static const char* CXXR::Promise::staticTypeName ( )
inlinestatic

The name by which this type is known in R.

Returns:
The name by which this type is known in R.
const char* CXXR::Promise::typeName ( ) const
virtual

Name within R of this type of object.

Returns:
the name by which this type of object is known within R.

Reimplemented from CXXR::RObject.

RObject* CXXR::Promise::value ( )
inline

Access the value of a Promise.

Returns:
pointer to the value of the Promise, or to Symbol::unboundValue() if it has not yet been evaluated.
const RObject* CXXR::Promise::valueGenerator ( ) const
inline

RObject to be evaluated by the Promise.

Returns:
const pointer to the RObject to be evaluated by the Promise.
void CXXR::Promise::visitReferents ( const_visitor v) const
virtual

Conduct a visitor to the nodes referred to by this one.

The referents of this node are those objects (derived from GCNode) designated by a GCEdge within this object.

Parameters:
vPointer to the visitor object.
Note:
If this method is reimplemented in a derived class, the reimplemented version must remember to invoke visitReferents() for the immediate base class of the derived class, to ensure that all referents of the object get visited. It is suggested that implementations set up stack-based pointers to all the referents of a node before visiting any of them; in that case, if the (recursive) visiting pushes the node out of the processor cache, there is no need to fetch it back in.

Reimplemented from CXXR::RObject.


The documentation for this class was generated from the following file: