const modifier

**Vertex34** · 09-27-2004

I am working my way through a c programming book and I am a little confused with the const modifier. It says in my book that after you declare a variable with the const modifier, the content of a variable cannot be changed. But if i run this program:

#include <stdio.h>

int main(int argc, char *argv[])
{
int const x = 10;
x = 54;
printf("%i",x);

system("PAUSE");
return 0;
}

It compiles fine and I was able to change the contents of the variable. The only thing that happend was my compiler (bloodshed dev c++) give me a warning about trying to change a read only variable. So is that the only purpose of the const modifier, to give you a warning if you try to change it?

**Hammer** · 09-27-2004

The compile is doing its job by warning you of the problem. Turn up the warning/error levels to get it to halt during the compile. The behaviour of the program when run is undefined. It would be allowable for the const variable to be put into read-only memory, any attempt to write to it would result in a crash. In your case, this isn't happening.... that's the magic of "undefined".

**xErath** · 09-27-2004

Extra: I recomend using

Code:

puts("Press any key...");getch();

instead of system("pause"); because it's portable, and it doesn't have to run a command line interpreter, which is more inefficient.

**Prelude** · 09-27-2004

>because it's portable
Bwahahaha!

>which is more inefficient
You also forgot safer. If pause happens to be a malicious program than you just hosed your system.

**itsme86** · 09-27-2004

puts("Press any key..."); getchar(); would be much better (read: portable).

**xErath** · 09-27-2004

Originally Posted by Prelude

>because it's portable
Bwahahaha!.

I didn't knew that old amiga or spectrum also have pause command lines...

**xErath** · 09-27-2004

Originally Posted by itsme86

puts("Press any key..."); getchar(); would be much better (read: portable).

getchar() waits for a character key... followed by enter (I think). getch() returns after any key press except keys like alt, ctrl, shift, caps lock, num lock, scroll lock, windows logo... bla bla bla.
e.g. getchar(); doesn't return the F** keys.

**itsme86** · 09-27-2004

http://www.delorie.com/djgpp/doc/libc/libc_383.html

Highlights are:

Code:

Syntax 
#include <conio.h>

int getch(void);

Portability
ANSI/ISO C No 
POSIX     No

**itsme86** · 09-27-2004

Originally Posted by xErath

getchar() waits for a character key... followed by enter (I think). getch() returns after any key press except keys like alt, ctrl, shift, caps lock, num lock, scroll lock, windows logo... bla bla bla.
e.g. getchar(); doesn't return the F** keys.

That really has nothing to do with the getchar() function. Putting the terminal in "raw" mode before calling getchar() will cause getchar() to return even key press as it's typed. In fact, I imagine that that's all getch() does is put the terminal in raw mode, call getchar(), and then return the terminal to its previous state.

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