8. Special object functions

Callback functions used to overload various builtin functions.

The functions can be grouped into a few sets:

• Object initialization and destruction.

  __INIT(), create(), _destruct()

• Unary operator overloading.

  `~(), `!(), _values(), cast(), _sizeof(), _indices(), __hash()

• Binary asymmetric operator overloading.

  `+(), ``+(), `-(), ``-(), `&(), ``&(), `|(), ``|(), `^(), ``^(), `<<(), ``<<(), `>>(), ``>>(), `*(), ``*(), `/(), ``/(), `%(), ``%()

• Binary symmetric operator overloading.

  The optimizer will make assumptions about the relations between these functions.

  `==(), _equal(), `<(), `>()

• Other binary operator overloading.

  `[](), `[]=(), `->(), `->=(), `+=(), `()()

• Overloading of other builtin functions.

  _is_type(), _sprintf(), _m_delete(), _get_iterator(), _search()

Although these functions are called from outside the object they exist in, they will still be used even if they are declared protected. It is in fact recommended to declare them protected, since that will hinder them being used for other purposes.

::

Inherit and variable initialization.

This function is generated automatically by the compiler. It's called just before lfun::create() when an object is instantiated.

It first calls any __INIT functions in inherited classes (regardless of modifiers on the inherits). It then executes all the variable initialization expressions in this class, in the order they occur.

This function can not be overloaded or blocked from executing.

lfun::create()

Low-level object creation callback.

This function is generated automatically by the compiler for inline classes that declare parameters. A call to it and its arguments are automatically added to user-supplied lfun::create()

This function is typically created implicitly by the compiler using the syntax:

class Foo(int foo) {
  int bar;
}

In the above case an implicit lfun::__create__() is created, and it's equivalent to:

class Foo {
  int foo;
  int bar;
  local protected void __create__(int foo)
  {
    this::foo = foo;
  }
}

Note also that in case lfun::create() does not exist, it will be created as an alias for this function.

This function did not exist in Pike 8.0 and earlier (where it was instead automatically inlined in lfun::create().

lfun::create(), lfun::__INIT()

Hashing callback.

The main caller of this function is predef::hash_value() or the low-level equivalent, which get called by various mapping operations when the object is used as index in a mapping.

It should return an integer that corresponds to the object in such a way that all values which lfun::`== considers equal to the object get the same hash value.

The function predef::hash does not return hash values that are compatible with this one.

It's assumed that this function is side-effect free.

lfun::`==, predef::hash_value()

Called by annotations()

Inherit in the current object to return the annotations for. If UNDEFINED or left out, this_program::this should be used (ie start at the current context and ignore any overloaded symbols).

Access permission override. One of the following:

Include nested annotations from the inherits.

annotations()

Get and set the value for an index atomically.

Index for which to get and set the value.

Value to set.

Returns the previous value at index index.

lfun::`->=(), lfun::`[]=(), atomic_get_set(), lfun::_m_delete(), lfun::`[](), lfun::`->()

Dispatch function for Serialization.deserialize().

Object to serialize. Always a context of the current object.

Function to be called once for every variable to serialize.

The deserializer function expects to be called with three arguments:

setter - Function that sets the symbol value.

symbol - The symbol name.

symbol_type - The type of the symbol.

A default implementation of lfun::_serialize() and lfun::_deserialize() is available in Serializer.Serializable.

lfun::_serialize(), Serializer.deserialize(), Serializer.Serializable()->_deserialize()

Object destruction callback.

This function is called right before the object is destructed. That can happen either through a call to predef::destruct(), when there are no more references to the object, or when the garbage collector discovers that it's part of a cyclic data structure that has become garbage.

A flag that tells why the object is destructed:

Objects are normally not destructed when a process exits, so _destruct functions aren't called then. Use atexit to get called when the process exits.

Regarding destruction order during garbage collection:

If an object is destructed by the garbage collector, it's part of a reference cycle with other things but with no external references. If there are other objects with _destruct functions in the same cycle, it becomes a problem which to call first.

E.g. if this object has a variable with another object which (directly or indirectly) points back to this one, you might find that the other object already has been destructed and the variable thus contains zero.

The garbage collector tries to minimize such problems by defining an order as far as possible:

• If an object A contains an lfun::_destruct and an object B does not, then A is destructed before B.

• If A references B single way, then A is destructed before B.

• If A and B are in a cycle, and there is a reference somewhere from B to A that is weaker than any reference from A to B, then A is destructed before B.

• If a cycle is resolved according to the rule above by ignoring a weaker reference, and there is another ambiguous cycle that would get resolved by ignoring the same reference, then the latter cycle will be resolved by ignoring that reference.

• Weak references (e.g. set with predef::set_weak_flag()) are considered weaker than normal references, and both are considered weaker than strong references.

• Strong references are those from objects to the objects of their lexically surrounding classes. There can never be a cycle consisting only of strong references. (This means the gc never destructs a parent object before all children have been destructed.)

An example with well defined destruct order due to strong references:

class Super {
  class Sub {
    protected void _destruct() {
      if (!Super::this)
        error ("My parent has been destructed!\n");
    }
  }
  Sub sub = Sub();
  protected void _destruct() {
    if (!sub)
      werror ("sub already destructed.\n");
  }
}

The garbage collector ensures that these objects are destructed in an order so that werror in Super is called and not error in Sub.

When the garbage collector calls lfun::_destruct, all accessible non-objects and objects without _destruct functions are still intact. They are not freed if the _destruct function adds external references to them. However, all objects with lfun::_destruct in the cycle are already scheduled for destruction and will therefore be destroyed even if external references are added to them.

lfun::create(), predef::destruct()

Recursive equality callback.

Is expected to return 1 if the current object is equal to arg, and 0 (zero) otherwise.

It's assumed that this function is side-effect free.

Note that this function may return different values at different times for the same argument due to the mutability of the object.

predef::equal(), lfun::`==()

Iterator creation callback.

Optional extra arguments as passed to get_iterator(). The implicit call from foreach() does not provide any arguments.

The returned predef::Iterator instance works as a cursor that references a specific item contained (in some arbitrary sense) in this one.

It's assumed that this function is side-effect free.

predef::Iterator, predef::get_iterator, predef::foreach()

List indices callback.

Expected to return an array with the valid indices in the object.

It's assumed that this function is side-effect free.

predef::indices(), lfun::_values(), lfun::_types(), ::_indices()

Type comparison callback.

Called by the cast operator to determine if an object simulates a basic type.

One of:

The following five shouldn't occurr, but are here for completeness:

Expected to return 1 if the object is to be regarded as a simulation of the type specified by basic_type.

The argument is currently a string with the name of the type, but might in the future be a value of the type type.

It's assumed that this function is side-effect free.

Called in Iterator objects by foreach (optional).

Returns the current index for an iterator, or UNDEFINED if the iterator doesn't point to any item. If this function is not present, the return value from lfun::_iterator_next() will be used.

If there's no obvious index set then the index is the current position in the data set, counting from 0 (zero).

predef::iterator_index(), lfun::_iterator_next(), lfun::_iterator_prev(), lfun::_iterator_value()

Called in Iterator objects by foreach.

Advances the iterator one step.

Iterators start at the position before the first element, and foreach calls this function repeatedly until it returns UNDEFINED.

Calling it again after it has returned UNDEFINED will typically cause it to restart the iteration with the first element (ie the start and end sentinel values are the same).

Returns UNDEFINED if there are no more elements in the iterator. Otherwise it may return any other value, which for convenience will be used as index and/or value in case there is no lfun::_iterator_index() and/or no lfun::_iterator_value().

This is the only function that is required by the iterator API.

predef::iterator_next(), lfun::_iterator_prev(), lfun::_iterator_index(), lfun::_iterator_value()

Step an iterator backwards.

Calling this function after it or _iterator_next() it has returned UNDEFINED will typically cause it to restart the iteration with the last element (ie the start and end sentinel values are the same).

Returns UNDEFINED if there are no more elements in the iterator. Otherwise it may return any other value, which for convenience may be used as index and/or value in case there is no lfun::_iterator_index() and/or no lfun::_iterator_value().

This function is an optional part of the iterator API and is not called directly by the runtime.

predef::iterator_prev(), lfun::_iterator_next(), lfun::_iterator_value(), lfun::_iterator_index()

Called in Iterator objects by foreach (optional).

Returns the current value for an iterator, or UNDEFINED if the iterator doesn't point to any item.

predef::iterator_value(), lfun::_iterator_next(), lfun::_iterator_prev(), lfun::_iterator_index()

Called by m_add().

lfun::_m_delete(), lfun::_m_clear()

Called by m_clear().

lfun::_m_delete(), lfun::_m_add()

Delete index callback.

predef::m_delete()

Called by random(). Typical use is when the object implements a ADT, when a call to this lfun should return a random member of the ADT or range implied by the ADT.

A RandomInterface()->random_string function that returns a string(8bit) of the specified length.

A RandomInterface()->random function.

predef::random(), RandomInterface

Called by reverse().

Search callback.

The arguments are sent straight from search(), and are as follows:

Value to search for.

The first position to search.

Optional extra arguments as passed to search().

predef::search()

Dispatch function for Serializer.serialize().

Object to serialize. Always a context of the current object.

Function to be called once for every variable to serialize.

The serializer function expects to be called with three arguments:

value - The value of the symbol.

symbol - The symbol name.

symbol_type - The type of the symbol.

A default implementation of lfun::_serialize() and lfun::_deserialize() is available in Serializer.Serializable.

lfun::_deserialize(), Serializer.serialize(), Serializer.Serializable()->_serialize()

Debug.size_object() callback.

Returns an approximation of the memory use in bytes for the object.

Debug.size_object(), lfun::_sizeof()

Size query callback.

Called by predef::sizeof() to determine the number of elements in an object. If this function is not present, the number of public symbols in the object will be returned.

Expected to return the number of valid indices in the object.

It's assumed that this function is side-effect free.

predef::sizeof()

Sprintf callback.

This method is called by predef::sprintf() to print objects. If it is not present, printing of the object will not be supported for any conversion-type except for the %O-conversion-type, which will output "object".

One of:

Conversion parameters. The following parameters may be supplied:

Is expected to return a string describing the object formatted according to conversion_type.

_sprintf() is currently not called for the following conversion-types:

This function might be called at odd times, e.g. before lfun::create has been called or when an error has occurred. The reason is typically that it gets called when a backtrace is being formatted to report an error. It should therefore be very robust and not make any assumptions about its own internal state, at least not when conversion_type is 'O'.

It's assumed that this function is side-effect free.

predef::sprintf()

Called by sqrt().

sqrt()

Called by sqrt when the square root of an object is requested.

_sqrt is not a real lfun, so it must not be defined as static.

predef::sqrt()

List types callback.

This callback is typically called via predef::types().

Expected to return an array with the types corresponding to the indices returned by lfun::_indices().

It's assumed that this function is side-effect free.

predef::types() was added in Pike 7.9.

predef::types(), lfun::_indices(), lfun::_values(), ::_types()

List values callback.

Expected to return an array with the values corresponding to the indices returned by lfun::_indices().

It's assumed that this function is side-effect free.

predef::values(), lfun::_indices(), lfun::_types(), ::_values()

Logical not callback.

Returns non-zero if the object should be evaluated as false, and 0 (zero) otherwise.

It's assumed that this function is side-effect free.

predef::`!()

Left side modulo callback.

It's assumed that this function is side-effect free.

lfun::``%(), predef::`%()

Left side bitwise and/intersection callback.

It's assumed that this function is side-effect free.

lfun::``&(), predef::`&()

Apply callback.

predef::`()

Left side multiplication/repetition/implosion callback.

It's assumed that this function is side-effect free.

lfun::``*(), predef::`*()

Called by predef::`**().

predef::`**(), lfun::``**()

Left side addition/concatenation callback.

This is used by predef::`+. It's called with the argument that follow this object in the argument list of the call to predef::`+. The returned value should be a new instance that represents the addition/concatenation between this object and the argument.

It's assumed that this function is side-effect free.

In versions of Pike prior to 8.1.10 this function could get called with multiple arguments.

lfun::``+(), lfun::`+=(), predef::`+()

Destructive addition/concatenation callback.

This is used by predef::`+. It's called with the argument that follow this object in the argument list of the call to predef::`+. It should update this object to represent the addition/concatenation between it and the argument. It should always return this object.

This function should only be implemented if lfun::`+() also is. It should only work as a more optimized alternative to that one, for the case when it's safe to change the object destructively and use it directly as the result.

This function is not an lfun for the += operator. It's only whether or not it's safe to do a destructive change that decides if this function or lfun::`+() is called; both the + operator and the += operator can call either one.

In versions of Pike prior to 8.1.10 this function could get called with multiple arguments.

lfun::`+(), predef::`+()

Negation and left side subtraction/set difference callback.

This is used by predef::`-. When called without an argument the result should be a new instance that represents the negation of this object, otherwise the result should be a new instance that represents the difference between this object and arg.

It's assumed that this function is side-effect free.

lfun::``-(), predef::`-()

Arrow index callback.

Symbol in context to access.

Context in the current object to start the search from. If UNDEFINED or left out, this_program::this is to be be used (ie start at the current context and ignore any overloaded symbols).

Access permission override. One of the following:

Returns the value at index if it exists, and UNDEFINED otherwise.

It's assumed that this function is side-effect free.

predef::`->(), ::`->()

Atomic get and set arrow index callback.

Symbol in context to change the value of.

The new value.

Context in the current object to index.

If UNDEFINED or left out, this_program::this is to be used (ie start at the current context and ignore any overloaded symbols).

Access permission override. One of the following:

This function is to set the value at symbol index of the current object to value.

Returns the previous value at symbol index of the current object.

In Pike 8.0 and earlier the return value of this function was ignored.

predef::`->=(), ::`->=(), lfun::`[]=(), lfun::_atomic_get_set()

Left side division/split callback.

It's assumed that this function is side-effect free.

lfun::``/(), predef::`/()

Less than test callback.

It's assumed that this function is side-effect free.

predef::`<()

Left side left shift callback.

It's assumed that this function is side-effect free.

lfun::``<<(), predef::`<<()

Equivalence test callback.

Is expected to return 1 if the current object is equivalent to arg (ie may be replaced with arg, with no semantic differences (disregarding the effects of destruct())), and 0 (zero) otherwise.

If this is implemented it may be necessary to implement lfun::__hash too. Otherwise mappings may hold several objects as indices which are duplicates according to this function. This may also affect various other functions that use hashing internally, e.g. predef::Array.uniq.

It's assumed that this function is side-effect free.

It's recommended to only implement this function for immutable objects, as otherwise stuff may get confusing when things that once were equivalent no longer are so, or the reverse.

predef::`==(), lfun::__hash

Greater than test callback.

It's assumed that this function is side-effect free.

predef::`>()

Left side right shift callback.

It's assumed that this function is side-effect free.

lfun::``>>(), predef::`>>()

Subrange callback.

It's assumed that this function is side-effect free.

predef::`[..]

Indexing callback.

For compatibility, this is also called to do subranges unless there is a `[..] in the class. See predef::`[..] for details.

It's assumed that this function is side-effect free.

predef::`[](), predef::`[..]

Atomic get and set index callback.

Index to change the value of.

The new value.

Context in the current object to index.

If UNDEFINED or left out, this_program::this is to be used (ie start at the current context and ignore any overloaded symbols).

Access permission override. One of the following:

This function is to set the value at index index of the current object to value.

Returns the previous value at index index of the current object.

In Pike 8.0 and earlier the return value of this function was ignored.

predef::`[]=(), lfun::`->=(), lfun::_atomic_get_set()

Left side exclusive or callback.

It's assumed that this function is side-effect free.

lfun::``^(), predef::`^()

Right side modulo callback.

It's assumed that this function is side-effect free.

lfun::`%(), predef::`%()

Right side bitwise and/intersection callback.

It's assumed that this function is side-effect free.

lfun::`&(), predef::`&()

Right side multiplication/repetition/implosion callback.

It's assumed that this function is side-effect free.

lfun::`*(), predef::`*()

Called by predef::`**().

predef::`**(), lfun::`**()

Right side addition/concatenation callback.

This is used by predef::`+. It's called with the sum of the arguments that precede this object in the argument list of the call to predef::`+. The returned value should be a new instance that represents the addition/concatenation between the argument and this object.

It's assumed that this function is side-effect free.

In versions of Pike prior to 8.1.10 this function could get called with multiple arguments.

lfun::`+(), predef::`+()

Right side subtraction/set difference callback.

This is used by predef::`-. The result should be a new instance that represents the difference between arg and this object.

It's assumed that this function is side-effect free.

lfun::`-(), predef::`-()

Right side division/split callback.

It's assumed that this function is side-effect free.

lfun::`/(), predef::`/()

Right side left shift callback.

It's assumed that this function is side-effect free.

lfun::`<<(), predef::`<<()

Right side right shift callback.

It's assumed that this function is side-effect free.

lfun::`>>(), predef::`>>()

Right side exclusive or callback.

It's assumed that this function is side-effect free.

lfun::`^(), predef::`^()

Right side bitwise or/union callback.

It's assumed that this function is side-effect free.

lfun::`|(), predef::`|()

Left side bitwise or/union callback.

It's assumed that this function is side-effect free.

lfun::``|(), predef::`|()

Complement/inversion callback.

It's assumed that this function is side-effect free.

predef::`~()

Value cast callback.

Type to cast to.

Expected to return the object value-casted (converted) to the type described by requested_type.

The argument is currently a string with the name of the type, but might in the future be a value of the type type.

Currently casting between object types is a noop.

If the returned value is not deemed to be of the requested type a runtime error may be thrown.

It's assumed that this function is side-effect free.

Object creation callback.

This function is called right after lfun::__INIT().

args are the arguments passed when the program was called.

If there exists an implicitly created lfun::__create__() its arguments will be prepended to args (affecting the prototype for lfun::create()), and a call to it will be prepended to the code of lfun::create().

In Pike 8.0 and earlier the code equivalent to lfun::__create__() was inlined at the beginning of lfun::create().

If this function does not exist, but lfun::__create__() does, it will instead be called directly.

lfun::__create__(), lfun::__INIT(), lfun::_destruct()

Symbols implicitly inherited from the virtual base class.

These symbols exist mainly to simplify implementation of the corresponding lfuns.

lfun::

Called by annotations()

Inherit in the current object to return the annotations for. If UNDEFINED or left out, this_program::this should be used (ie start at the current context and ignore any overloaded symbols).

Access permission override. One of the following:

Include nested annotations from the inherits.

Builtin function to list the annotations (if any) of the identifiers of an object. This is useful when lfun::_annotations has been overloaded.

annotations(), lfun::_annotations()

Context in the current object to start the list from. If UNDEFINED or left out, this_program::this will be used (ie start at the current context and ignore any overloaded symbols).

Access permission override. One of the following:

Builtin function to list the identifiers of an object. This is useful when lfun::_indices has been overloaded.

::_values(), ::_types(), ::`->()

Context in the current object to start the list from. If UNDEFINED or left out, this_program::this will be used (ie start at the current context and ignore any overloaded symbols).

Access permission override. One of the following:

Builtin function to list the types of the identifiers of an object. This is useful when lfun::_types has been overloaded.

::_indices(), ::_values(), ::`->()

Context in the current object to start the list from. If UNDEFINED or left out, this_program::this will be used (ie start at the current context and ignore any overloaded symbols).

Access permission override. One of the following:

Builtin function to list the values of the identifiers of an object. This is useful when lfun::_values has been overloaded.

::_indices(), ::_types(), ::`->()

Builtin arrow operator.

Symbol in context to access.

Context in the current object to start the search from. If UNDEFINED or left out, this_program::this will be used (ie start at the current context and ignore any overloaded symbols).

Access permission override. One of the following:

This function indexes the current object with the string index. This is useful when the arrow operator has been overloaded.

::`->=()

Builtin atomic arrow get and set operator.

Symbol in context to change the value of.

The new value.

Context in the current object to start the search from. If UNDEFINED or left out, this_program::this will be used (ie start at the current context and ignore any overloaded symbols).

Access permission override. One of the following:

This function indexes the current object with the string index, and sets it to value. This is useful when the arrow set operator has been overloaded.

Returns the previous value at index index of the current object.

::`->()

Symbols specific for restartable functions.

this_function, Restartable functions

This symbol is only valid in restartable functions and evaluates to the current restartable function (ie the function that will continue the current function from the next restart point (ie next yield() or continue return). For generator functions this is the same value as the value that was returned by the "outer" (ie generator) function.

This differs from predef::this_function which evaluates to the "outer" function.

predef::this_function, predef::this, predef::this_object()