Re: HW8

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Posted by Stu on July 18, 2000 at 20:11:31:

In Reply to: HW8 posted by Mike on July 18, 2000 at 19:17:07:

:
: Ok, I got no quams with this. However, in class you also defined a new variable like this (unless I copied wrong):

: NodeType a;
: a = new NodeType;

You must have copied it wrong (we'll assume so) :)
Basics are:

// C-code snippet

NodeType a; // a is a 8-byte NodeType struct
NodeType* pa; // pa is an uninitialized 4-byte
// ptr that _can_ point to a
// NodeType struct but doesn't yet.
pa = NULL ; // Now pa just points to address 0.
// Still doesn't point at a valid
// NodeType in memory somewhere yet.
pa = new NodeType ; // Now it points to a real
// NodeType struct.

pa->i = 42 ; // Now we can safely
pa->Next = NULL ; // dereference it. Happy fun!

// Fini la snippet

: The book does something similar:

: NodeType* a = NULL;
: a = new NodeType;

: However, I can't get either to compile.

This code works fine given the original struct def. You must have mistyped something. Post an error message if you can't get it to work.

: However, this does [work]:

: NodeType* Head = new NodeType;

Yep. Same syntax as:
NodeType* a = new NodeType ;

: I was wondering, can I assume that the above is a Head that points to nothing (not even NULL)?

The 'new' operator returns a pointer. When you say:
NodeType* Head = new NodeType;
... you are assigning to 'Head', the pointer to the memory that was just allocated for the new NodeType struct you requested.

: Also, is were you find NULL?
Heh - couldn't see The NULL symbol is defined in several standard header files. Try highlighting it and right-clicking and choosing 'go to definition' in the menu selection and it'll jump to whatever header defined it. I think stdio.h is one that includes it, probably stdlib and a hundred others too. You can define it yourself if you're feeling frisky. Looks like:

#define NULL 0

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: : : : Ok, I got no quams with this. However, in class you also defined a new variable like this (unless I copied wrong): : : NodeType a; : : a = new NodeType; : You must have copied it wrong (we'll assume so) :) : Basics are: : // C-code snippet : NodeType a; // a is a 8-byte NodeType struct : NodeType* pa; // pa is an uninitialized 4-byte : // ptr that _can_ point to a : // NodeType struct but doesn't yet. : pa = NULL ; // Now pa just points to address 0. : // Still doesn't point at a valid : // NodeType in memory somewhere yet. : pa = new NodeType ; // Now it points to a real : // NodeType struct. : pa->i = 42 ; // Now we can safely : pa->Next = NULL ; // dereference it. Happy fun! : // Fini la snippet : : The book does something similar: : : NodeType* a = NULL; : : a = new NodeType; : : However, I can't get either to compile. : This code works fine given the original struct def. You must have mistyped something. Post an error message if you can't get it to work. : : However, this does [work]: : : NodeType* Head = new NodeType; : Yep. Same syntax as: : NodeType* a = new NodeType ; : : I was wondering, can I assume that the above is a Head that points to nothing (not even NULL)? : The 'new' operator returns a pointer. When you say: : NodeType* Head = new NodeType; : ... you are assigning to 'Head', the pointer to the memory that was just allocated for the new NodeType struct you requested. : : Also, is <stdio.h> were you find NULL? : Heh - couldn't see <stdio.h> until just now because the greater-than/less-than signs got interpreted as an html tag. Buggy bboard code! Riot! Riot! Etc... : The NULL symbol is defined in several standard header files. Try highlighting it and right-clicking and choosing 'go to definition' in the menu selection and it'll jump to whatever header defined it. I think stdio.h is one that includes it, probably stdlib and a hundred others too. You can define it yourself if you're feeling frisky. Looks like: : #define NULL 0

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