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+'\"
+'\" Copyright (c) 1996-1997 Sun Microsystems, Inc.
+'\"
+'\" See the file "license.terms" for information on usage and redistribution
+'\" of this file, and for a DISCLAIMER OF ALL WARRANTIES.
+'\" 
+'\" SCCS: @(#) @(#) Object.3 1.10 97/07/22 11:40:10
+'\" 
+.so man.macros
+.TH Tcl_Obj 3 8.0 Tcl "Tcl Library Procedures"
+.BS
+.SH NAME
+Tcl_NewObj, Tcl_DuplicateObj, Tcl_IncrRefCount, Tcl_DecrRefCount, Tcl_IsShared \- manipulate Tcl objects
+.SH SYNOPSIS
+.nf
+\fB#include <tcl.h>\fR
+.sp
+Tcl_Obj *
+\fBTcl_NewObj\fR()
+.sp
+Tcl_Obj *
+\fBTcl_DuplicateObj\fR(\fIobjPtr\fR)
+.sp
+\fBTcl_IncrRefCount\fR(\fIobjPtr\fR)
+.sp
+\fBTcl_DecrRefCount\fR(\fIobjPtr\fR)
+.sp
+int
+\fBTcl_IsShared\fR(\fIobjPtr\fR)
+.sp
+\fBTcl_InvalidateStringRep\fR(\fIobjPtr\fR)
+.SH ARGUMENTS
+.AS Tcl_Obj *objPtr in
+.AP Tcl_Obj *objPtr in
+Points to an object;
+must have been the result of a previous call to \fBTcl_NewObj\fR.
+.BE
+
+.SH INTRODUCTION
+.PP
+This man page presents an overview of Tcl objects and how they are used.
+It also describes generic procedures for managing Tcl objects.
+These procedures are used to create and copy objects,
+and increment and decrement the count of references (pointers) to objects.
+The procedures are used in conjunction with ones
+that operate on specific types of objects such as
+\fBTcl_GetIntFromObj\fR and \fBTcl_ListObjAppendElement\fR.
+The individual procedures are described along with the data structures
+they manipulate.
+.PP
+Tcl's \fIdual-ported\fR objects provide a general-purpose mechanism
+for storing and exchanging Tcl values.
+They largely replace the use of strings in Tcl.
+For example, they are used to store variable values,
+command arguments, command results, and scripts.
+Tcl objects behave like strings but also hold an internal representation
+that can be manipulated more efficiently.
+For example, a Tcl list is now represented as an object
+that holds the list's string representation
+as well as an array of pointers to the objects for each list element.
+Dual-ported objects avoid most runtime type conversions.
+They also improve the speed of many operations
+since an appropriate representation is immediately available.
+The compiler itself uses Tcl objects to
+cache the instruction bytecodes resulting from compiling scripts.
+.PP
+The two representations are a cache of each other and are computed lazily.
+That is, each representation is only computed when necessary,
+it is computed from the other representation,
+and, once computed, it is saved.
+In addition, a change in one representation invalidates the other one.
+As an example, a Tcl program doing integer calculations can
+operate directly on a variable's internal machine integer
+representation without having to constantly convert
+between integers and strings.
+Only when it needs a string representing the variable's value,
+say to print it,
+will the program regenerate the string representation from the integer.
+Although objects contain an internal representation,
+their semantics are defined in terms of strings:
+an up-to-date string can always be obtained,
+and any change to the object will be reflected in that string
+when the object's string representation is fetched.
+Because of this representation invalidation and regeneration,
+it is dangerous for extension writers to access
+\fBTcl_Obj\fR fields directly.
+It is better to access Tcl_Obj information using
+procedures like \fBTcl_GetStringFromObj\fR.
+.PP
+Objects are allocated on the heap
+and are referenced using a pointer to their \fBTcl_Obj\fR structure.
+Objects are shared as much as possible.
+This significantly reduces storage requirements
+because some objects such as long lists are very large.
+Also, most Tcl values are only read and never modified.
+This is especially true for procedure arguments,
+which can be shared between the caller and the called procedure.
+Assignment and argument binding is done by
+simply assigning a pointer to the value. 
+Reference counting is used to determine when it is safe to
+reclaim an object's storage.
+.PP
+Tcl objects are typed.
+An object's internal representation is controlled by its type.
+Seven types are predefined in the Tcl core
+including integer, double, list, and bytecode.
+Extension writers can extend the set of types
+by using the procedure \fBTcl_RegisterObjType\fR .
+
+.SH "THE TCL_OBJ STRUCTURE"
+.PP
+Each Tcl object is represented by a \fBTcl_Obj\fR structure
+which is defined as follows.
+.CS
+typedef struct Tcl_Obj {
+	int \fIrefCount\fR;
+	char *\fIbytes\fR;
+	int \fIlength\fR;
+	Tcl_ObjType *\fItypePtr\fR;
+	union {
+		long \fIlongValue\fR;
+		double \fIdoubleValue\fR;
+		VOID *\fIotherValuePtr\fR;
+		struct {
+			VOID *\fIptr1\fR;
+			VOID *\fIptr2\fR;
+		} \fItwoPtrValue\fR;
+	} \fIinternalRep\fR;
+} Tcl_Obj;
+.CE
+The \fIbytes\fR and the \fIlength\fR members together hold
+an object's string representation,
+which is a \fIcounted\fR or \fIbinary string\fR
+that may contain binary data with embedded null bytes.
+\fIbytes\fR points to the first byte of the string representation.
+The \fIlength\fR member gives the number of bytes.
+The byte array must always have a null after the last byte,
+at offset \fIlength\fR;
+this allows string representations that do not contain nulls
+to be treated as conventional null-terminated C strings.
+C programs use \fBTcl_GetStringFromObj\fR to get
+an object's string representation.
+If \fIbytes\fR is NULL,
+the string representation is invalid.
+.PP
+An object's type manages its internal representation.
+The member \fItypePtr\fR points to the Tcl_ObjType structure
+that describes the type.
+If \fItypePtr\fR is NULL,
+the internal representation is invalid.
+.PP
+The \fIinternalRep\fR union member holds
+an object's internal representation.
+This is either a (long) integer, a double-precision floating point number,
+a pointer to a value containing additional information
+needed by the object's type to represent the object,
+or two arbitrary pointers.
+.PP
+The \fIrefCount\fR member is used to tell when it is safe to free
+an object's storage.
+It holds the count of active references to the object.
+Maintaining the correct reference count is a key responsibility
+of extension writers.
+Reference counting is discussed below
+in the section \fBSTORAGE MANAGEMENT OF OBJECTS\fR.
+.PP
+Although extension writers can directly access
+the members of a Tcl_Obj structure,
+it is much better to use the appropriate procedures and macros.
+For example, extension writers should never
+read or update \fIrefCount\fR directly;
+they should use macros such as
+\fBTcl_IncrRefCount\fR and \fBTcl_IsShared\fR instead.
+.PP
+A key property of Tcl objects is that they hold two representations.
+An object typically starts out containing only a string representation:
+it is untyped and has a NULL \fItypePtr\fR.
+An object containing an empty string or a copy of a specified string
+is created using \fBTcl_NewObj\fR or \fBTcl_NewStringObj\fR respectively.
+An object's string value is gotten with \fBTcl_GetStringFromObj\fR
+and changed with \fBTcl_SetStringObj\fR.
+If the object is later passed to a procedure like \fBTcl_GetIntFromObj\fR
+that requires a specific internal representation,
+the procedure will create one and set the object's \fItypePtr\fR.
+The internal representation is computed from the string representation.
+An object's two representations are duals of each other:
+changes made to one are reflected in the other.
+For example, \fBTcl_ListObjReplace\fR will modify an object's
+internal representation and the next call to \fBTcl_GetStringFromObj\fR
+will reflect that change.
+.PP
+Representations are recomputed lazily for efficiency.
+A change to one representation made by a procedure
+such as \fBTcl_ListObjReplace\fR is not reflected immediately
+in the other representation.
+Instead, the other representation is marked invalid
+so that it is only regenerated if it is needed later.
+Most C programmers never have to be concerned with how this is done
+and simply use procedures such as \fBTcl_GetBooleanFromObj\fR or
+\fBTcl_ListObjIndex\fR.
+Programmers that implement their own object types
+must check for invalid representations
+and mark representations invalid when necessary.
+The procedure \fBTcl_InvalidateStringRep\fR is used
+to mark an object's string representation invalid and to
+free any storage associated with the old string representation.
+.PP
+Objects usually remain one type over their life,
+but occasionally an object must be converted from one type to another.
+For example, a C program might build up a string in an object
+with repeated calls to \fBTcl_StringObjAppend\fR,
+and then call \fBTcl_ListObjIndex\fR to extract a list element from
+the object.
+The same object holding the same string value
+can have several different internal representations
+at different times.
+Extension writers can also force an object to be converted from one type
+to another using the \fBTcl_ConvertToType\fR procedure.
+Only programmers that create new object types need to be concerned
+about how this is done.
+A procedure defined as part of the object type's implementation
+creates a new internal representation for an object
+and changes its \fItypePtr\fR.
+See the man page for \fBTcl_RegisterObjType\fR
+to see how to create a new object type.
+
+.SH "EXAMPLE OF THE LIFETIME OF AN OBJECT"
+.PP
+As an example of the lifetime of an object,
+consider the following sequence of commands:
+.CS
+\fBset x 123\fR
+.CE
+This assigns to \fIx\fR an untyped object whose
+\fIbytes\fR member points to \fB123\fR and \fIlength\fR member contains 3.
+The object's \fItypePtr\fR member is NULL.
+.CS
+\fBputs "x is $x"\fR
+.CE
+\fIx\fR's string representation is valid (since \fIbytes\fR is non-NULL)
+and is fetched for the command.
+.CS
+\fBincr x\fR
+.CE
+The \fBincr\fR command first gets an integer from \fIx\fR's object
+by calling \fBTcl_GetIntFromObj\fR.
+This procedure checks whether the object is already an integer object.
+Since it is not, it converts the object
+by setting the object's \fIinternalRep.longValue\fR member
+to the integer \fB123\fR
+and setting the object's \fItypePtr\fR
+to point to the integer Tcl_ObjType structure.
+Both representations are now valid.
+\fBincr\fR increments the object's integer internal representation
+then invalidates its string representation
+(by calling \fBTcl_InvalidateStringRep\fR)
+since the string representation
+no longer corresponds to the internal representation.
+.CS
+\fBputs "x is now $x"\fR
+.CE
+The string representation of \fIx\fR's object is needed
+and is recomputed.
+The string representation is now \fB124\fR.
+and both representations are again valid.
+
+.SH "STORAGE MANAGEMENT OF OBJECTS"
+.PP
+Tcl objects are allocated on the heap and are shared as much as possible
+to reduce storage requirements.
+Reference counting is used to determine when an object is
+no longer needed and can safely be freed.
+An object just created by \fBTcl_NewObj\fR or \fBTcl_NewStringObj\fR
+has \fIrefCount\fR 0.
+The macro \fBTcl_IncrRefCount\fR increments the reference count
+when a new reference to the object is created.
+The macro \fBTcl_DecrRefCount\fR decrements the count
+when a reference is no longer needed and,
+if the object's reference count drops to zero, frees its storage.
+An object shared by different code or data structures has
+\fIrefCount\fR greater than 1.
+Incrementing an object's reference count ensures that
+it won't be freed too early or have its value change accidently.
+.PP
+As an example, the bytecode interpreter shares argument objects
+between calling and called Tcl procedures to avoid having to copy objects.
+It assigns the call's argument objects to the procedure's
+formal parameter variables.
+In doing so, it calls \fBTcl_IncrRefCount\fR to increment
+the reference count of each argument since there is now a new
+reference to it from the formal parameter.
+When the called procedure returns,
+the interpreter calls \fBTcl_DecrRefCount\fR to decrement
+each argument's reference count.
+When an object's reference count drops to zero,
+\fBTcl_DecrRefCount\fR reclaims its storage.
+Most command procedures do not have to be concerned about
+reference counting since they use an object's value immediately
+and don't retain a pointer to the object after they return.
+However, if they do retain a pointer to an object in a data structure,
+they must be careful to increment its reference count
+since the retained pointer is a new reference.
+.PP
+Command procedures that directly modify objects
+such as those for \fBlappend\fR and \fBlinsert\fR must be careful to
+copy a shared object before changing it.
+They must first check whether the object is shared
+by calling \fBTcl_IsShared\fR.
+If the object is shared they must copy the object
+by using \fBTcl_DuplicateObj\fR;
+this returns a new duplicate of the original object
+that has \fIrefCount\fR 0.
+If the object is not shared,
+the command procedure "owns" the object and can safely modify it directly.
+For example, the following code appears in the command procedure
+that implements \fBlinsert\fR.
+This procedure modifies the list object passed to it in \fIobjv[1]\fR
+by inserting \fIobjc-3\fR new elements before \fIindex\fR.
+.CS
+listPtr = objv[1];
+if (Tcl_IsShared(listPtr)) {
+	listPtr = Tcl_DuplicateObj(listPtr);
+}
+result = Tcl_ListObjReplace(interp, listPtr, index, 0, (objc-3), &(objv[3]));
+.CE
+As another example, \fBincr\fR's command procedure
+must check whether the variable's object is shared before
+incrementing the integer in its internal representation.
+If it is shared, it needs to duplicate the object
+in order to avoid accidently changing values in other data structures.
+
+.SH "SEE ALSO"
+Tcl_ConvertToType, Tcl_GetIntFromObj, Tcl_ListObjAppendElement, Tcl_ListObjIndex, Tcl_ListObjReplace, Tcl_RegisterObjType
+
+.SH KEYWORDS
+internal representation, object, object creation, object type, reference counting, string representation, type conversion