Pointers and references: Difference between revisions
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===Pointers=== |
===Pointers=== |
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Pointers are declared like so, optionally with the type or program they will point to: |
Pointers are declared like so, optionally with the type or program they will point to: |
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<syntaxhighlight lang=" |
<syntaxhighlight lang="cobolfree">01 ptr USAGE IS POINTER TO Some-Type. |
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01 prog-ptr USAGE IS PROGRAM-POINTER TO "some-program".</syntaxhighlight> |
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<code>USAGE POINTER</code> data items are used in conjunction with <code>BASED</code> data items, with <code>ALLOCATE</code> optionally giving a pointer the address of the allocated memory. Pointers can also be used to free allocated memory, which will cause the pointer to be set to <code>NULL</code>. |
<code>USAGE POINTER</code> data items are used in conjunction with <code>BASED</code> data items, with <code>ALLOCATE</code> optionally giving a pointer the address of the allocated memory. Pointers can also be used to free allocated memory, which will cause the pointer to be set to <code>NULL</code>. |
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<syntaxhighlight lang=" |
<syntaxhighlight lang="cobolfree">01 heap-item PICTURE IS X, BASED. |
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... |
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PROCEDURE DIVISION. |
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ALLOCATE heap-item RETURNING ptr |
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*> ... |
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<code>USAGE PROGRAM-POINTER</code> data items are used to point to programs and their entry points. |
<code>USAGE PROGRAM-POINTER</code> data items are used to point to programs and their entry points. |
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{{works with|OpenCOBOL}} |
{{works with|OpenCOBOL}} |
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{{works with|Visual COBOL}} |
{{works with|Visual COBOL}} |
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<syntaxhighlight lang=" |
<syntaxhighlight lang="cobolfree">SET prog-ptr TO ENTRY "some-program"</syntaxhighlight> |
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Both types of pointer support basic pointer arithmetic. |
Both types of pointer support basic pointer arithmetic. |
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<syntaxhighlight lang=" |
<syntaxhighlight lang="cobolfree">SET ptr1 UP BY 10 |
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SET ptr2 DOWN BY LENGTH OF foo</syntaxhighlight> |
SET ptr2 DOWN BY LENGTH OF foo</syntaxhighlight> |
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Pointers can also be set to point to where other pointers are pointing or to other pointers themselves. |
Pointers can also be set to point to where other pointers are pointing or to other pointers themselves. |
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<syntaxhighlight lang=" |
<syntaxhighlight lang="cobolfree">SET ptr1 TO ptr2 *> ptr1 points to where ptr2 points |
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SET ptr2 TO ADDRESS OF ptr3 *> ptr2 points to ptr3</syntaxhighlight> |
SET ptr2 TO ADDRESS OF ptr3 *> ptr2 points to ptr3</syntaxhighlight> |
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To alter the value pointed to by a pointer, the <code>SET</code> statement is needed once again and is used to set the address of <code>BASED</code> or <code>LINKAGE SECTION</code> data items, which can then be used to modify the data. |
To alter the value pointed to by a pointer, the <code>SET</code> statement is needed once again and is used to set the address of <code>BASED</code> or <code>LINKAGE SECTION</code> data items, which can then be used to modify the data. |
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<syntaxhighlight lang=" |
<syntaxhighlight lang="cobolfree">SET ADDRESS OF foo TO ptr |
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MOVE "bar" TO foo</syntaxhighlight> |
MOVE "bar" TO foo</syntaxhighlight> |
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===References=== |
===References=== |
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Object references are declared like so, optionally with the class/interface they will reference: |
Object references are declared like so, optionally with the class/interface they will reference: |
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<syntaxhighlight lang=" |
<syntaxhighlight lang="cobolfree">01 obj USAGE IS OBJECT-REFERENCE "some-object".</syntaxhighlight> |
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They contain either a reference to an object or <code>NULL</code>. |
They contain either a reference to an object or <code>NULL</code>. |
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They are initialised using by invoking a class constructor, and set using the <code>SET</code> statement. |
They are initialised using by invoking a class constructor, and set using the <code>SET</code> statement. |
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<syntaxhighlight lang=" |
<syntaxhighlight lang="cobolfree">INVOKE SomeClass "new" RETURNING obj-ref |
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SET another-obj-ref TO obj-ref</syntaxhighlight> |
SET another-obj-ref TO obj-ref</syntaxhighlight> |
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