const keyword in C and C++

Given the following:

char my_string[] = "12345";

You can create a pointer to this char array with, or without, the "const" keyword. The four options are these:

char *my_ptr1 = my_string;

char *my_ptr2 const = my_string;

const char *my_ptr3 = my_string;

const char *my_ptr4 const = my_string;

These all point to the '1' in "12345".

For "my_ptr1", any of the following operations are legal:

my_ptr1 = my_string + 1; // now points to '2' in "12345"

*my_ptr1 = '8'; // '1' in "12345" is changed to an '8', making "82345"

(*my_ptr1)++; // '1' in "12345" is changed to an '2', and my_ptr1 now points to the second '2' in "22345"

For "my_ptr2", the following are legal:

my_ptr2 = my_string + 1; // now points to '2' in "12345"

my_ptr2++; // also points to '2' in "12345"

However, you cannot change the value of "my_string" in any way. This means that

*my_ptr1 = '8';

would not change the string to "82345", but instead your compiler would inform you that this is an illegal operation.

For "my_ptr3", the following is legal:

*my_ptr3 = '8'; // '1' in "12345" is changed to '8', making "82345"

However, changing the address where it pointed to would be illegal:

my_ptr3 = my_string + 1;

my_ptr3++;

Having the "const" identifier AFTER the pointer declaration means that the pointer can't move or be changed, although the data CAN be.

Finally, there's "my_ptr4". You cannot change the address of my_ptr4 (same restrictions as my_ptr3 has), nor can you change the data that it is pointing at (same restrictions as my_ptr2).

None of these are legal operations:

my_ptr4 = my_string + 1;

my_ptr4[1] = '8';

If you want to see some of this in action, check out the following code. Change line 6

char *my_ptr = my_string;

to include the "const" keyword in various places as shown above to see the different kinds of compiler warnings/errors you get. (Tested with i686-apple-darwin8-gcc-4.0.1 (GCC) 4.0.1 (Apple Computer, Inc. build 5367) compiler.)

#include

int main()

{

char my_string[] = "12345";

char *my_ptr = my_string; // you could also use "&my_string[0]", which is the same thing

// Before any modifications

printf("my_string: \"%s\"\n", my_string);

printf("my_ptr is pointing to the \'%c\' in \"%s\"\n", *my_ptr, my_string);

// This will change my_string to equal "18345"

my_ptr[1] = '8';

printf("\n");

printf("my_string now equals: \"%s\"\n", my_string);

// This will make my_ptr point to the '8' in "18345"

my_ptr++;

printf("\n");

printf("my_ptr is now pointing to the \'%c\' in \"%s\"\n", *my_ptr, my_string);

// This will change the '8' in "18345"

(*my_ptr)++;

printf("\n");

printf("my_string now equals: \"%s\"\n", my_string);

printf("my_ptr is now pointing to the \'%c\' in \"%s\"\n", *my_ptr, my_string);

return 0;

}

KQlives on the Mac

I've been out of the programming loop for a while.  Two kids can do that to you.

I wanted to download and play with KQlives on my iMac.  The download from Sourceforge provided an outdated version, but which ran.  The CVS version gave me all the current updates, but wouldn't run (and compile).

My iMac isn't configured for programming: it's got XCode, a few programming projects I've worked on, DarwinPorts (very nice, by the way), and a few libraries to run Allegro-built games.

When I started, I was informed that my 'autoreconf' was out-of-date.  I upgraded to the most recent version via DarwinPorts, then got a bunch of warnings stating that a bunch of stuff was deprecated.  Lovely.

I don't remember how, but after hammering away at it for a good day or two, I finally got it to create a Makefile for me.

I ran that, and found that I didn't have the Allegro4, Lua, or DuMB libraries installed.  I grabbed them from DarwinPorts, but to my chagrin, it didn't give me any header files or sources: just the precompiled binaries.

I went off and downloaded the library sources, installed, and then was able to compile KQ (yay!).

Trying to RUN kq gave me problems.  It created a blank window, then complained that it was missing "misc.pcx".  I saw a few other reports like this on Google from a few years ago, but no resolution.  I found the answer, so I wanted to create this blog in case someone else happens across it.

I found out what was happening by adding a line of code that told me where, exactly, the datafile was trying to be found (the "mpcx.dat" datafile specifically).  Instead of being under the current directory (or even under ./data/mpcx.dat like I expected), it was looking for it under "./Contents/Resources/data/mpcx.dat"  So I simply created that directory, copied everything from ./data into that directory, and ran it.  It gave me the opening credits!  Of course, it died shortly after, complaining about some scripts and whatnot, but it's the same situation: it needs "./Contents/Resources/scripts/" and all of the other "important" subdirectories (like music, etc.) to be moved/copied under that.

If I get this up and working, I may even be able to work on some more code and see if I can't resurrect a few things for the kqlives project.  Kids- and time-willing, of course.