Undefined values: Difference between revisions

Line 135:

int main()

{

int junk, *junkp;

/* Print an unitialized variable! */

printf("junk: %d\n", junk);

/* Follow a pointer to unitialized memory! */

junkp = malloc(sizeof *junkp);

if (junkp)

printf("*junkp: %d\n", *junkp);

return 0;

}</lang>

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There is no undefined value in Déjà Vu. Instead, trying to access an undefined variable raises an exception.

<lang dejavu>try:

bogus

catch name-error:

!print "There is *no* :bogus in the current context"

return

!print "You won't see this."</lang>

If you need to declare a local variable, but don't have a value for it yet, there is the standard function <code>undef</code>, which raises an exception when called but is an actual value that can be passed around and assigned to names.

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=={{header|Raku}}==

(formerly Perl 6)

Raku has "interesting" values of undef, but unlike Perl 5, doesn't actually have a value named <tt>undef</tt>. Instead, several very different meanings of undefinedness are distinguished. First, <tt>Nil</tt> represents the absence of a value. The absence of a value cannot be stored. Instead, an attempt to assign <tt>Nil</tt> to a storage location causes that location to revert to its uninitialized state, however that is defined.

<lang perl6>my $x; $x = 42; $x = Nil; say $x.WHAT; # prints Any()</lang>

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Done

</pre>

=={{header|Rust}}==

All variables in Rust must have defined type and value.

For the representation of a value which might not be present, the standard library offers wrapper type <code>Option</code>.

<code>Option</code> has two variants: <code>Some</code> (holding a value) and <code>None</code> (indicating the absence of any value):

<code>Option</code> is a regular <code>enum</code> type and there is nothing special about it, except for its broad usage.

Rust supports raw pointers.

Pointers are used for interoperability with other languages and for low-level unsafe operations.

Safe Rust code can't use pointers as effective use of a pointer (dereferencing it) requires <code>unsafe</code> block.

A pointer may have a ''null'' value:

<lang Rust>use std::ptr;

let p: *const i32 = ptr::null();

assert!(p.is_null());</lang>

=={{header|Scala}}==