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/const.html | 212 ++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 212 insertions(+) create mode 100644 reference/C/SYNTAX/const.html (limited to 'reference/C/SYNTAX/const.html') diff --git a/reference/C/SYNTAX/const.html b/reference/C/SYNTAX/const.html new file mode 100644 index 0000000..3ca3683 --- /dev/null +++ b/reference/C/SYNTAX/const.html @@ -0,0 +1,212 @@ + +The const keyword. + + + + + +

+ +

The const keyword.

+ +

+ +The const keyword is used to create a read only variable. Once initialised, +the value of the variable cannot be changed but can be used just like +any other variable. + +

const syntax

+ + +

+
+        main()
+        {
+            const float pi = 3.14; 
+        }
+

+ +

+The const keyword is used as a qualifier to the following data types - +int float char double struct. +

+ + +

+
+        const int   degrees = 360; 
+        const float pi      = 3.14; 
+        const char  quit    = 'q'; 
+

+ +

const and pointers.

+ +Consider the following example. +

+ + +

+
+        void Func(const char *Str);
+
+        main()
+        {
+            char *Word;
+
+            Word = (char *) malloc(20);
+
+            strcpy(Word, "Sulphate");
+          
+            Func(Word);
+        }
+        
+        void Func(const char *Str)
+        {
+        }
+

+ +

+The const char *Str tells the compiler that the DATA the +pointer points too is const. This means, Str can be changed +within Func, but *Str cannot. As a copy of the pointer is passed to Func, +any changes made to Str are not seen by main.... + +

+ + +

+
+            --------
+           | Str    |  Value can be changed
+            -----|--
+                 |
+                 |
+                 V
+            --------
+           | *Str   | Read Only - Cannot be changed.
+            --------
+

+ +

Geeky Stuff

+ +It is still possible to change the contents of a 'const' variable. +Consider this program it creates +a const variable and then changes its value by accessing the data by +another name. +

+I am not sure if this applies to all compilers, but, +you can place the 'const' after the datatype, for example: +

+ + +

+
+        int   const degrees = 360; 
+        float const pi      = 3.14; 
+        char  const quit    = 'q'; 
+

+ +

+are all valid in 'gcc'. +

More Geeky Stuff

+ +What would you expect these to do? +

+ + +

+
+        main()
+        {
+            const char * const Variable1;
+            char const * const Variable2;
+        };
+

+ +

+These both make the pointer and the data read only. +Here are a few more examples. +

+ + +

+
+  const int   Var;           /* Var is constant */
+  int const   Var;           /* Ditto */
+  int * const Var;           /* The pointer is constant, 
+                              * the data its self can change. */
+  const int * Var;           /* Var can not be changed. */  
+                           
+

+ +