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struct within a struct

Posted on 2010-11-29
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Last Modified: 2012-05-10
hi there,

i came across this struct code and i noticed that it is declaring itself inside it owns struct.

for example:
i have a struct called abc. and inside this abc struct, there is a declaration of struct abc with a next pointer pointing to it.

struct abc {
        char a[20];
        long b;
        struct abc * next;
}

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may i know how is this possible and what is the purpose for this?

thanks in advance.
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Question by:gagajanice
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by:phoffric
ID: 34235670
This is not actually a struct within a struct (or nested struct). next is just a pointer to a struct abc. One common application for this is a single linked list. See, for example, the code in this question:
   http://www.experts-exchange.com/Programming/Languages/C/Q_23404167.html
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by:DonConsolio
DonConsolio earned 50 total points
ID: 34235672
"next" is a pointer to a "struct abc" object.

The compiler knows the size and behavior of a pointer and can
insert it without knowing all the internals of the "abc" struct.
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phoffric earned 150 total points
ID: 34235685
Here are short tutorials on single linked lists:
    http://richardbowles.tripod.com/cpp/linklist/linklist.htm
    http://www.cprogramming.com/tutorial/lesson15.html

Notice that the code has the pointer to the struct.
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Assisted Solution

by:themrrobert
themrrobert earned 50 total points
ID: 34235783
The reason is so that it can be a linked list, so that you can iterate through easily by changing the ref after your done using it.


i believe this is the best simplest answer to your question
struct* myStruct;

while(struct->next!=null) {
//do stuff
struct = struct->next; //automatically moves to next item
}

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Author Closing Comment

by:gagajanice
ID: 34236080
thank you everyone for the explaination.

phoffric, thanks for the url. very useful to me.
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Expert Comment

by:DeepuAbrahamK
ID: 34236101
if you try using this:

struct abc {
        char a[20];
        long b;
        struct abc  next; //<-----without the pointer (*)
}

 you will get an error: 'next' uses undefined struct 'abc' since it is not declared.

Have a look at nested structs:
http://www.exforsys.com/tutorials/c-plus-plus/structure-in-c-part-ii.html
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