C++ problem

[Joey]

ok ive had this problem for months, but i havent worked on it. what is wrong with this code? its supposed to make the sides of a box in dos. this is the part thats supposed to do the center lines:

// build the center line
blanks[0] = border[7];
i = rgt - lft;
blanks[ i ] = border[3];
blanks[ i+1 ] = 0;


//write the center lines
i = top + 1;
while ( i < btm )
{
gotoxy( lft, i );
cprintf( blanks );
i++;
}




this isnt the whole code, this is just the portion that is supposed to do the center lines i guess.

[Joey]

ok then. here is a .txt file with the whole file. whats wrong?

[attachment deleted by admin]

[beyond infinity lazy]

ok, joey, so you have provided the whole source code, thus faciliating some quick checks. for I have no c++ compiler at this pc at work, I canna do a test compile, but from what I see, you might have some small mistake in it:

It is this place here in your main function:

box_draw( 5, 5, 45, 15, "?????? ", 62, 23 );

Here is the declaration of your box_draw:

void box_draw( int lft, int top, int rgt, int btm,
char* border, int outline_clr, int contents_clr );

Mark the following: if you define somewhere a pointer, you have to pass an adress to it bofore you can use it further, else it points to nowhere in silicium hell.

This char* border pointer you have to provide with the adress of a string instead of a string for it is a POINTER!

Know about pointers and how to use the *-operator with them? It's nearby the same as with the brackets[] in nasm.

I'll tell you a bit about it:

with int *p you define a pointer to any integer var.

with p=&integervar you pass the adress of any integer to the pointer.

with p++/p-- you increment/decrement the adress of the pointer by the size of the datatype it points to. p. ex. for our int-pointer p p would then increase by 4 bytes.

with *p you access the content of the variable p points to.

so for your problem:

you ought define a string in your main function:

Code: Select all

 
char border[8]="12345678"

you then call box_draw in this way:

box_draw( 5, 5, 45, 15, border, 62, 23 );

the parameter char *border is provided with the adress of the string you have defined, because the name of an array is also it's adress.(equal to &border[0])

it should do what you expect it to do then.

stay safe

[Tim]

Beyond: some good reasoning in your post there. However, it had one small but fatal flaw: a string literal is a pointer.

Therefore these two are equally valid:

Code: Select all

char str1[] = "hello\n";
char *str2 = "hello\n";
printf(str1);
printf(str2);

Joey: I can't see anything obviously wrong with your code, but then again, it won't compile on my machine (I don't have a <conio.h> that is compatible with yours).

[sonneveld]

One small thing.

I wouldn't recommend using

Code: Select all

printf(str1);

and instead use

Code: Select all

printf("%s", str1);

for the simple reason that if a user can set what's in str1 and puts in a bunch of %n's (writes the number of characters to an int pointer), he could manipulate printf to write data to strange locations.

- Nick

[beyond infinity lazy]

aye tim, that's correct.

's been an error of mine blinded by kernel coding and so forth. Sometimes one traps himself and overlooks the small essential thingses.

[Whatever5k]

Therefore these two are equally valid:

Code: Select all

char str1[] = "hello\n";
char *str2 = "hello\n";
printf(str1);
printf(str2);

That's right for this example, but still: be aware of such pointers. Following example will generate a Segmentation Fault:

Code: Select all

char *str = "Hello\n";

printf("%s", str);
strcpy(str, "Bla\n");
printf("%s", str);

The pointer str does not reference to a specified address. Chaning char *str to char str[] will make the program working perfectly.
Just wanted to point out to this thing

[Tim]

[tt]char *str = "Hello";[/tt] does reference a specific address, but the compiler has probably put it in a read-only section within the program's executable. For example, the gcc option [tt]-fwritable-strings[/tt] would let you do [tt]strcpy("Hello", "World");[/tt].

[Joey]

all of you just confused the hell out of me. so what do i change to fix it? what tim said?

[Tim]

No, the last few posts were red herrings.

I can't see anything wrong with your code, except a small mistake here:

Code: Select all

//write the center line
i + top + 1;

I assume that first '+' is meant to be '='.

I get this output:
[tt]+-------------------------+write the bottom[/tt]
Edit: replaced linedraw characters with - and + (YaBB doesn't like HTML &1234; codes).