33 lines
1.0 KiB
QBasic
33 lines
1.0 KiB
QBasic
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10 REM A strict pointer is a distinct declared kind. Assignment always copies;
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20 REM POINT is the only way to share, so a program that never writes it can
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30 REM never be surprised by aliasing.
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40 TYPE NODE
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50 COUNT#
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60 TAIL@ AS PTR TO NODE
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70 END TYPE
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80 DIM N1@ AS NODE
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90 DIM N2@ AS NODE
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100 DIM N3@ AS NODE
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110 N1@.COUNT# = 10
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120 N2@.COUNT# = 20
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130 N3@.COUNT# = 30
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140 REM A TYPE may refer to itself only through PTR TO, which is what makes a
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150 REM list possible at all -- by value it would have no finite size.
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160 POINT N1@.TAIL@ AT N2@
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170 POINT N2@.TAIL@ AT N3@
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180 DIM WALK@ AS PTR TO NODE
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190 POINT WALK@ AT N1@
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200 PRINT WALK@->COUNT#
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210 WALK@ = WALK@->TAIL@
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220 PRINT WALK@->COUNT#
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230 WALK@ = WALK@->TAIL@
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240 PRINT WALK@->COUNT#
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250 REM An unbound pointer is NOTHING, not a crash.
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260 PRINT WALK@->TAIL@
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270 REM Writing through a pointer changes what it points at.
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280 POINT WALK@ AT N1@
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290 WALK@->COUNT# = 99
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300 PRINT N1@.COUNT#
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310 REM And the whole list renders, following pointers as it goes.
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320 PRINT N1@
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