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Ingy döt Net 2023-07-01 11:58:00 -04:00
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Task/Polymorphic-copy/ALGOL-68/polymorphic-copy.alg Normal file
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BEGIN
# Algol 68 doesn't have classes and inheritence as such, however structures #
# can contain procedures and different instances of a structure can have #
# different versons of the procedure #
# this allows us to simulate inheritence by creating structure instances #
# with different procedures #
# the following declares a type (MODE) HORSE with two constructors, one for #
# a standard horse and one for a zebra (the "derived class"). #
# The standard horse has its print procedure set to thw print horse #
# procedure (the "base class" method). zebras get the print zebra procedure #
# (the "derived class" method). A convenience operator (PRINT) is defined #
# to simplify calling the horse/zebra print method of its horse parameter #
# = this PRINT operator is independent of which actual print method the #
# horse has - it just saved typeing "( print OF h )( h )" everywhere we #
# need to call the print method (euivalent to h.print(h) in e.g. C, Java, #
# etc.) #
# "class" #
MODE HORSE = STRUCT( STRING name, PROC(HORSE)VOID print );
# constructors #
PROC new horse = ( STRING name )HORSE: ( name, print horse );
PROC new zebra = ( STRING name )HORSE: ( name, print zebra );
# print methods: one for a standard horse and one for a zebra #
PROC print horse = ( HORSE h )VOID: print( ( "horse: ", name OF h ) );
PROC print zebra = ( HORSE h )VOID: print( ( "zebra: ", name OF h ) );
# print operator #
OP PRINT = ( HORSE h )VOID: ( print OF h )( h );
# declare and construct some horses and zebras #
HORSE h1 := new horse( "silver blaze" );
HORSE z1 := new zebra( "stripy" );
HORSE z2 := new zebra( "second zebra" );
# show their values #
PRINT h1; print( ( newline ) );
PRINT z1; print( ( newline ) );
PRINT z2; print( ( newline ) );
print( ( "----", newline ) );
# change the second zebra to be a copy of the first zebra #
z2 := z1;
PRINT z1; print( ( newline ) );
PRINT z2; print( ( newline ) );
print( ( "----", newline ) );
# change the name of the first zebra leaving z2 unchanged #
name OF z1 := "ed";
PRINT z1; print( ( newline ) );
PRINT z2; print( ( newline ) )
END