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const Keyword

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Started by WiseElben July 27, 2003 09:31 PM

5 comments, last by WiseElben 20 years, 8 months ago

WiseElben

250

Author

July 27, 2003 09:31 PM


char a_string[64] = "Hello";
 
const char* buff1 = a_string;
char* const buff2 = a_string;
 
buff1 = "Hi there"; // It's totally legal to set buff1 to another pointer to constant data

*buff2 = 'w'; // It's totally legal to change any value pointed to by buff2

At GT's const Keyword tutorial, it states that it is legal. I don't understand why it is legal. Visit my game programming website at www.wiseelben.com [edited by - WiseElben on July 27, 2003 10:37:29 PM] [edited by - WiseElben on July 27, 2003 10:37:51 PM] [edited by - WiseElben on July 27, 2003 10:45:45 PM] [edited by - WiseElben on July 27, 2003 10:52:47 PM] [edited by - WiseElben on July 27, 2003 10:53:04 PM]

WiseElben

250

Author

July 27, 2003 09:45 PM

Wait... maybe I understand...

So buff1 = "Hi there"; is changing a_string, not buff1, right?
And *buff2 = ''w''; is changing a_string also?

No, wait... Actually, I''m lost again...

Visit my game programming website at www.wiseelben.com

Greatwolf

125

July 27, 2003 09:48 PM

oh, it''s simple. For buff1 it''s legal because the pointer itself is not constant. So that means we can have the pointer point to something else, which is what the code''s doing for buff1.

For buff2, the pointer is constant but the value pointed to is not constant. We won''t be able to redirect buff2 to point to something else but we can change the value buff2 is pointing to.

Does that make any sense?

--{You fight like a dairy farmer!}

spock

217

July 27, 2003 09:49 PM

The placement of const in the declaration determines what''s constant and what is variable. For buff1 the constness applies to the characters pointed at (in which case it''s OK to change the pointer) and for buff2 it applies to the pointer itself (which means it''s OK to change the things being pointed at but not the pointer).

Both could be made const with a declaration such as

const char* const buff3 = a_string;

and neither assignment would be legal for buff3.

Scatter/Gather thoughts

WiseElben

250

Author

July 27, 2003 09:58 PM

quote:Original post by spock
The placement of const in the declaration determines what''s constant and what is variable. For buff1 the constness applies to the characters pointed at (in which case it''s OK to change the pointer).

So that means the constantness is the a_string?

buff1 = "Hi there";

So what does that change? The memory of buff1 to somewhere called "Hi there"?

Visit my game programming website at www.wiseelben.com

spock

217

July 27, 2003 10:11 PM

quote:Original post by WiseElben
So that means the constantness is the a_string?

buff1 = "Hi there";

So what does that change? The memory of buff1 to somewhere called "Hi there"?

That changes the pointer buff1 so that it points to wherever the memory for the string literal "Hi there" is.

Scatter/Gather thoughts

WiseElben

250

Author

July 27, 2003 10:13 PM

Oh, I see. Thanks.

Visit my game programming website at www.wiseelben.com