A pointer to persistent storage assigned to transient memory is valid only until the end of the outermost transaction in which the assignment occurred, unless objectstore::retain_persistent_addresses() is used. In addition, a pointer to storage in one database assigned to storage in another database is valid only until the end of the outermost transaction in which the assignment occurred, unless os_database::allow_external_pointers() or os_segment::allow_external_pointers() is used.
os_references, in contrast, are always valid across transaction boundaries, as well as across databases.
Once the object referred to by a reference is deleted, use of the reference accesses arbitrary data and might cause a segmentation violation. But see os_reference_protected.
part *a_part = ... ;
os_reference part_ref = a_part;
When an os_reference is cast to pointer-to-referent-type (that is, pointer to the type of object referred to by the reference), os_reference::operator void*() is implicitly invoked, returning a valid pointer to the object referred to by the os_reference.
printf("%d\n", (part*)(part_ref)->part_id);
In some cases involving multiple inheritance, comparing two references has a different result from comparing the corresponding pointers. For example, for == comparisons, if the referent type of one operand is a nonleftmost base class of the referent type of the other operand, the result is always 1. This is because comparing references never results in the pointer adjustment described in Section 10.3c of the C++ Annotated Reference Manual.Each instance of this class stores a relative pathname to identify the referent database. The pathname is relative to the lowest common directory in the pathnames of the referent database and the database containing the reference. For example, if a reference stored in /A/B/C/db1 refers to data in /A/B/D/db2, the lowest common directory is A/B, so the relative pathname ../../D/db2 is used.
This means that if you copy a database containing a reference, the reference in the copy and the reference in the original might refer to different databases. To change the database a reference refers to, you can use the ObjectStore utility oschangedbref. See ObjectStore Management.
The types os_int32 and os_boolean, used throughout this manual, are each defined as a signed 32-bit integer type. The type os_unsigned_int32 is defined as an unsigned 32-bit integer type.
All ObjectStore programs must include the header file <ostore/ostore.hh>.
char* dump(const char* db_str) const;Returns a heap-allocated text string representing the specified reference. However, unlike the string returned by the char * os_reference::dump(void) method, this string does not contain an absolute database path. The returned string is intended to be used as the dump_str parameter of an os_reference load method of the form load(const char* dump_str, os_database* db). It is the responsibility of the caller of load to ensure that the db parameter passed to the load method is the same as the database of the dumped reference. It is the user's responsibility to delete the returned string when finished using the string.
This operation is useful in those applications in which you do not want the overhead of storing the absolute database path in the dumped strings.
os_database *get_database() const;Returns a pointer to the database containing the object referred to by the specified reference.
char* get_database_key(const char* dump_str);Returns a heap-allocated string containing the database_key component of the string dump_str. dump_str must have been generated using the dump operation. Otherwise, the exception err_reference_syntax is raised. It is the user's responsibility to delete the returned string when finished using the string.
os_database *get_open_database() const;Returns a pointer to the database containing the object referred to by the specified os_reference. Opens the database.
static os_typespec *get_os_typespec();Returns an os_typespec* for the class os_reference.
os_unsigned_int32 hash() const;Returns an integer suitable for use as a hash table key. The value returned is always the same for an os_reference to a given referent.
void load(const char*);If the specified char* points to a string generated from a reference with os_reference::dump(), calling this function makes the specified reference refer to the same object referred to by the reference used to generate the string.
void load(const char* dump_str, const os_database* db);The dump_str parameter is assumed to be the result of a call to a compatible os_Reference dump method. It is the responsibility of the caller of load to ensure that the db parameter passed to the load method is the same as the database of the originally dumped reference.
The loaded reference refers to the same object as the os_Reference used to dump the string as long as the db parameter is the same as the database of the dumped reference.
The exception err_reference_syntax is raised if the dump_str is not in the expected format or if the dump_str was dumped from a protected reference.
operator void*() const;Returns the valid pointer for which the specified reference is a substitute.
os_reference &operator=(const os_reference&);Establishes the referent of the right operand as the referent of the left operand.
os_reference &operator=(const void*);Establishes the object pointed to by the right operand as the referent of the left operand.
os_boolean operator ==(os_reference const&) const;Returns 1 if the arguments have the same referent; returns 0 otherwise.
os_boolean operator !=(os_reference const&) const;Returns 1 if the arguments have different referents; returns 0 otherwise.
os_boolean operator <(os_reference const&) const;If the first and second arguments refer to elements of the same array or one beyond the end of the array, a return value of 1 indicates that the referent of the first argument precedes the referent of the second, and a return value of 0 indicates that it does not. Otherwise the results are undefined.
os_boolean operator >(os_reference const&) const;If the first and second arguments refer to elements of the same array or one beyond the end of the array, a return value of 1 indicates that the referent of the first argument follows the referent of the second, and a return value of 0 indicates that it does not. Otherwise the results are undefined.
os_boolean operator >=(os_reference const&) const;If the first and second arguments refer to elements of the same array or one beyond the end of the array, a return value of 1 indicates that the referent of the first argument follows or is the same as the referent of the second, and a return value of 0 indicates that it does not. Otherwise the results are undefined.
os_boolean operator <=(os_reference const&) const;If the first and second arguments refer to elements of the same array or one beyond the end of the array, a return value of 1 indicates that the referent of the first argument precedes or is the same as the referent of the second, and a return value of 0 indicates that it does not. Otherwise the results are undefined.
os_reference(const void*);Constructs a reference to substitute for the specified void*.
void* resolve() const;Returns the valid void* for which the specified reference is a substitute.
Updated: 03/31/98 17:25:09