path: root/doc/Object.3

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.TH Tcl_Obj 3 8.5 Tcl "Tcl Library Procedures"
.so man.macros
.BS
.SH NAME
Tcl_NewObj, Tcl_DuplicateObj, Tcl_IncrRefCount, Tcl_DecrRefCount, Tcl_IsShared, Tcl_InvalidateStringRep \- manipulate Tcl values
.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
.AP Tcl_Obj *objPtr in
Points to a value;
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 values (called \fBTcl_Obj\fRs for
historical reasons) and how they are used.
It also describes generic procedures for managing Tcl values.
These procedures are used to create and copy values,
and increment and decrement the count of references (pointers) to values.
The procedures are used in conjunction with ones
that operate on specific types of values 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 values 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 values behave like strings but also hold an internal representation
that can be manipulated more efficiently.
For example, a Tcl list is now represented as a value
that holds the list's string representation
as well as an array of pointers to the values for each list element.
Dual-ported values 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 values 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 values 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 value will be reflected in that string
when the value'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 and \fBTcl_GetString\fR.
.PP
Values are allocated on the heap
and are referenced using a pointer to their \fBTcl_Obj\fR structure.
Values are shared as much as possible.
This significantly reduces storage requirements
because some values 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 a value's storage.
.PP
Tcl values are typed.
A value's internal representation is controlled by its type.
Several types are predefined in the Tcl core
including integer, double, list, and bytecode.
Extension writers can extend the set of types
by defining their own \fBTcl_ObjType\fR structs.
.SH "THE TCL_OBJ STRUCTURE"
.PP
Each Tcl value is represented by a \fBTcl_Obj\fR structure
which is defined as follows.
.PP
.CS
typedef struct Tcl_Obj {
    int \fIrefCount\fR;
    char *\fIbytes\fR;
    int \fIlength\fR;
    const Tcl_ObjType *\fItypePtr\fR;
    union {
        long \fIlongValue\fR;
        double \fIdoubleValue\fR;
        void *\fIotherValuePtr\fR;
        Tcl_WideInt \fIwideValue\fR;
        struct {
            void *\fIptr1\fR;
            void *\fIptr2\fR;
        } \fItwoPtrValue\fR;
        struct {
            void *\fIptr\fR;
            unsigned long \fIvalue\fR;
        } \fIptrAndLongRep\fR;
    } \fIinternalRep\fR;
} \fBTcl_Obj\fR;
.CE
.PP
The \fIbytes\fR and the \fIlength\fR members together hold
a value's UTF-8 string representation,
which is a \fIcounted string\fR not containing null bytes (UTF-8 null
characters should be encoded as a two byte sequence: 192, 128.)
\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 byte after the last data byte,
at offset \fIlength\fR;
this allows string representations
to be treated as conventional null-terminated C strings.
C programs use \fBTcl_GetStringFromObj\fR and \fBTcl_GetString\fR to get
a value's string representation.
If \fIbytes\fR is NULL,
the string representation is invalid.
.PP
A value'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
a value'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 value's type to represent the value, a Tcl_WideInt
integer, two arbitrary pointers, or a pair made up of an unsigned long
integer and a pointer.
.PP
The \fIrefCount\fR member is used to tell when it is safe to free
a value's storage.
It holds the count of active references to the value.
Maintaining the correct reference count is a key responsibility
of extension writers.
Reference counting is discussed below
in the section \fBSTORAGE MANAGEMENT OF VALUES\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 values is that they hold two representations.
A value typically starts out containing only a string representation:
it is untyped and has a NULL \fItypePtr\fR.
A value containing an empty string or a copy of a specified string
is created using \fBTcl_NewObj\fR or \fBTcl_NewStringObj\fR respectively.
A value's string value is gotten with
\fBTcl_GetStringFromObj\fR or \fBTcl_GetString\fR
and changed with \fBTcl_SetStringObj\fR.
If the value is later passed to a procedure like \fBTcl_GetIntFromObj\fR
that requires a specific internal representation,
the procedure will create one and set the value's \fItypePtr\fR.
The internal representation is computed from the string representation.
A value's two representations are duals of each other:
changes made to one are reflected in the other.
For example, \fBTcl_ListObjReplace\fR will modify a value's
internal representation and the next call to \fBTcl_GetStringFromObj\fR
or \fBTcl_GetString\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 value types
must check for invalid representations
and mark representations invalid when necessary.
The procedure \fBTcl_InvalidateStringRep\fR is used
to mark a value's string representation invalid and to
free any storage associated with the old string representation.
.PP
Values usually remain one type over their life,
but occasionally a value must be converted from one type to another.
For example, a C program might build up a string in a value
with repeated calls to \fBTcl_AppendToObj\fR,
and then call \fBTcl_ListObjIndex\fR to extract a list element from
the value.
The same value holding the same string value
can have several different internal representations
at different times.
Extension writers can also force a value to be converted from one type
to another using the \fBTcl_ConvertToType\fR procedure.
Only programmers that create new value types need to be concerned
about how this is done.
A procedure defined as part of the value type's implementation
creates a new internal representation for a value
and changes its \fItypePtr\fR.
See the man page for \fBTcl_RegisterObjType\fR
to see how to create a new value type.
.SH "EXAMPLE OF THE LIFETIME OF A VALUE"
.PP
As an example of the lifetime of a value,
consider the following sequence of commands:
.PP
.CS
\fBset x 123\fR
.CE
.PP
This assigns to \fIx\fR an untyped value whose
\fIbytes\fR member points to \fB123\fR and \fIlength\fR member contains 3.
The value's \fItypePtr\fR member is NULL.
.PP
.CS
\fBputs "x is $x"\fR
.CE
.PP
\fIx\fR's string representation is valid (since \fIbytes\fR is non-NULL)
and is fetched for the command.
.PP
.CS
\fBincr x\fR
.CE
.PP
The \fBincr\fR command first gets an integer from \fIx\fR's value
by calling \fBTcl_GetIntFromObj\fR.
This procedure checks whether the value is already an integer value.
Since it is not, it converts the value
by setting the value's \fIinternalRep.longValue\fR member
to the integer \fB123\fR
and setting the value's \fItypePtr\fR
to point to the integer Tcl_ObjType structure.
Both representations are now valid.
\fBincr\fR increments the value'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.
.PP
.CS
\fBputs "x is now $x"\fR
.CE
.PP
The string representation of \fIx\fR's value is needed
and is recomputed.
The string representation is now \fB124\fR
and both representations are again valid.
.SH "STORAGE MANAGEMENT OF VALUES"
.PP
Tcl values are allocated on the heap and are shared as much as possible
to reduce storage requirements.
Reference counting is used to determine when a value is
no longer needed and can safely be freed.
A value 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 value is created.
The macro \fBTcl_DecrRefCount\fR decrements the count
when a reference is no longer needed and,
if the value's reference count drops to zero, frees its storage.
A value shared by different code or data structures has
\fIrefCount\fR greater than 1.
Incrementing a value's reference count ensures that
it will not be freed too early or have its value change accidentally.
.PP
As an example, the bytecode interpreter shares argument values
between calling and called Tcl procedures to avoid having to copy values.
It assigns the call's argument values 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 a value's reference count drops less than or equal to zero,
\fBTcl_DecrRefCount\fR reclaims its storage.
Most command procedures do not have to be concerned about
reference counting since they use a value's value immediately
and do not retain a pointer to the value after they return.
However, if they do retain a pointer to a value 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 values
such as those for \fBlappend\fR and \fBlinsert\fR must be careful to
copy a shared value before changing it.
They must first check whether the value is shared
by calling \fBTcl_IsShared\fR.
If the value is shared they must copy the value
by using \fBTcl_DuplicateObj\fR;
this returns a new duplicate of the original value
that has \fIrefCount\fR 0.
If the value is not shared,
the command procedure
.QW "owns"
the value 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 value passed to it in \fIobjv[1]\fR
by inserting \fIobjc-3\fR new elements before \fIindex\fR.
.PP
.CS
listPtr = objv[1];
if (\fBTcl_IsShared\fR(listPtr)) {
    listPtr = \fBTcl_DuplicateObj\fR(listPtr);
}
result = Tcl_ListObjReplace(interp, listPtr, index, 0,
        (objc-3), &(objv[3]));
.CE
.PP
As another example, \fBincr\fR's command procedure
must check whether the variable's value is shared before
incrementing the integer in its internal representation.
If it is shared, it needs to duplicate the value
in order to avoid accidentally changing values in other data structures.
.SH "SEE ALSO"
Tcl_ConvertToType(3), Tcl_GetIntFromObj(3), Tcl_ListObjAppendElement(3), Tcl_ListObjIndex(3), Tcl_ListObjReplace(3), Tcl_RegisterObjType(3)
.SH KEYWORDS
internal representation, value, value creation, value type,
reference counting, string representation, type conversion