From 7e0f021a9aec35fd8e6725e87e3313b101d26f5e Mon Sep 17 00:00:00 2001 From: Tobias Klauser Date: Sun, 27 Jan 2008 11:37:44 +0100 Subject: Initial import (2.0.2-6) --- reference/C/SYNTAX/struct.html | 392 +++++++++++++++++++++++++++++++++++++++++ 1 file changed, 392 insertions(+) create mode 100644 reference/C/SYNTAX/struct.html (limited to 'reference/C/SYNTAX/struct.html') diff --git a/reference/C/SYNTAX/struct.html b/reference/C/SYNTAX/struct.html new file mode 100644 index 0000000..6465ee6 --- /dev/null +++ b/reference/C/SYNTAX/struct.html @@ -0,0 +1,392 @@ +STRUCT keyword + + + + + + +

STRUCT keyword

+ +

Structure basics. +
Structure membership. +
Pointers to structures. +
Array of structures. +
The bottom draw.. +
C++ extensions +

+ + +

Structure basics

+ +struct is used to declare a new +data-type. +Basically this means +grouping variables together. For example, a struct data type could be used +to declare the format of the following file.

+ +

+ + +

          
+   Jo		Loss		Maths		A   
+   Harry	Carpenter	English 	A
+   Billy	King		Maths		C
+

+ +

+ +The records above could be described in a struct as follows: +

+ + +

   
+
+   struct
+   {
+       char cname[8];
+       char sname[16];   
+       char exam[16];
+       char grade;
+   } record;
+
+

+ +

+The statement above declares a variable called record with 4 members +called cname, sname, exam, grade. The structure as a whole can be +referred to as record and a member can be referenced as +record.exam +

+ +Structures can be declared in various forms... +

+ +

+	struct x {int a; int b; int c;};		/* declaration	*/
+	struct 	 {int a; int b; int c;} z;
+	struct x z;
+

+All the examples above are structure declarations, +

The first gives x as a 'structure tag' - this is optional. +
The first and second declare the members of the structure. +
Second and third give z this is the variable that assumes the +structure type. +

+ + +

Structure membership

+ +We can access individual members of a structure with the . operator.

+For example to assign a value, we can enter: +

+ + +

+	
+   struct x {int a; int b; int c;};   
+
+   main()
+   {
+      struct x z;
+     
+      z.a = 10;
+      z.b = 20;
+      z.c = 30;
+   }
+   
+

+ +

+And to retrieve a value from a structure member: +

+ + +

+
+   struct x
+   {
+      int a;
+      int b;
+      int c;
+   };
+
+   main()
+   {
+      struct x z;
+	
+      z.a = 10;
+      z.a++;
+
+      printf(" first member is %d \n", z.a);   
+   }
+   
+

+ +

Pointers to structures

+ +Fast path to an explanation of the -> operator. +

+ +All that we have discussed so far has been OK but runs into problems +when structures have to be moved between functions for the following +reasons. +

If the structure is large it is more effiecent to pass a +pointer to the structure +instead of the structure its self. This technic is also used to pass +pointers to arrays between +functions. +
+
When passing a structure to a function, you actually pass a COPY of the +structure. Therefore it is not possible to change the values of members +within the structure as the copy is destroyed when the function ends. +

+So how does it all work? Here is an example. (make your browser W-I-D-E so +you can see the two examples). +

+ + +

+					|
+					|
+   struct x {int a; int b; int c;} ;	| struct x {int a; int b; int c;} ;
+					|
+   void function(struct x);		| void function(struct x *);
+					|
+   main()			       	| main()
+   {					| {
+     struct x z;                        |   struct x z, *pz;	     /* 3 */
+					|   pz = &z;		     /* 4 */
+     z.a = 10;		/* 1 */		|   z.a = 10;
+     z.a++;				|   z.a++;
+					|
+     function(z);	/* 2 */	        | function(pz);		     /* 5 */
+   }					| }
+					|
+   void function( struct x z)		| void function(struct x * pz)
+   {					| {			     /* 6 */
+     printf(" first member %d \n", z.a);|   printf(" first member %d \n", (*pz).a);   
+   }					| }
+					|
+

+ +

+Here is the annotation. +

Give a structure member a value. +
Pass a COPY of the whole structure to the function. +
Define 'pz' a pointer to a structure of type 'x'. +
Put the address of 'z' into 'pz'. 'pz' now POINTS to 'z'. +PLEASE NOTE. 'z' is defined to reserve memory equal to the size of the +structure. 'pz' only holds an address so will be 4 bytes long. +
Pass the pointer into the function. +
Print the value of the member 'a'. +

+ + +The (*pz).a syntax is used a great deal in C and it was decided to create +a short hand for it. So: + +

+	(*pz).a   ==  pz->a
+

+ +Here is the final picture. +

+
+   /*************************************************************************/
+
+   struct x {int a; int b; int c;} ;	/* Declare the structure.	    */
+
+   void function(struct x * );		/* Declare the function.	    */
+
+   /*************************************************************************/
+
+   main() 
+   {  
+					/* Declare two variables.
+					 * z == type struct x
+					 * pz == a pointer to type struct x
+					 */
+     struct x z, *pz; 			
+
+     pz = &z;        			/* put the address of 'z' into 'pz' */
+     z.a = 10;				/* initialize z.a		    */
+     z.a++;				/* Increment z.a		    */
+
+					/* print the contents of 'z.a'
+					 * using the pointer 'pz'	    */
+   
+     printf(" first member before the function call %d \n", pz->a);
+
+					/* Call 'function' passing the 
+					 * pointer 'pz'			    */
+     function(pz);  			
+
+					/* Print the NEW value of 'z.a'
+					 * using three different notations  */
+     printf(" first member after the function call %d \n", pz->a);
+     printf(" first member after the function call %d \n", (*pz).a);
+     printf(" first member after the function call %d \n", z.a);
+
+   }
+
+   /*************************************************************************/
+   
+   void function(struct x * pz)
+   { 
+					/* Print the value of 'z.a' by
+					 * referencing the pointer 'pz'
+					 * which holds the address of 'z'   */
+     printf(" first member inside the function %d \n", pz->a);
+      
+					/* Increment the value of 'z.a'
+					 * this is the source location
+					 * in memory.			    */
+     pz->a++;
+     
+   }
+
+   /*************************************************************************/
+  
+
+

+ + +

The Bottom Draw

+ +Finally, here is a little feature that allows you to save a little +space. +

+ + +

+
+  main()
+  {
+    struct Flags
+    {
+      unsigned int Online  :1;   
+      unsigned int Mounted :1;
+    }
+
+    struct Flags TapeInfo;
+
+    TapeInfo.Online  = 1;
+    TapeInfo.Mounted = 0;
+  }
+  
+

+ +

+The :1 tells the compiler that only 1 byte is +required for Online and Mounted. There are a few points to +note about this though. +

You may expect the compiler to reserve 2 bytes for the structure, it +actually reserves one word (usually 4 bytes) as this is the smallest unit +that can be reserved, the remaining 2 bytes are unavailable. +This is still better than the 2 words that wold normally get reserved. +
You can put any number into the variable, if the number is too large +to fit, the high order bits are lost without warning. +
Only signed int, unsigned int, int support this syntax. +

+ +

Examples

+ + +This is the most basic struct example I could think of. +
+ + + +Using structure elements, and passing them into a function. +
+ + + +Passing a whole structure to a function. This performs a copy of the +structure so the same rules apply as for int etc. +Pointers to structures can be passed but I have not got to them yet.... +
+ + + +Define and use an array of structures. +
+ + + +Here is a struct problem for you. + + + +

STRUCT keyword

Structure basics

Structure membership

Pointers to structures

The Bottom Draw

Examples

See Also: