allocating dynamic memory

[Andreas Lassmann]

hi there, i've got a problem:
can i create a dynamic array like this?

pMap = new char[mapX][mapY];

gcc (my compiler) sais, it's wrong...
i know that dynamic memory is more often used in this way:

pMap = new char[mapX];

but i need a second element...
can somebody help?

 

[Howard]

hi there, i've got a problem:
can i create a dynamic array like this?

pMap = new char[mapX][mapY];

gcc (my compiler) sais, it's wrong...
i know that dynamic memory is more often used in this way:

pMap = new char[mapX];

but i need a second element...
can somebody help?

One way is to do this:

char** pMap = new char*[mapY]; // does char* need () around it?
for (int i = 0; i < mapY; ++i)
pMap = new char[mapX];

and later,

for (int i = 0; i < mapY; ++i)
delete [] pMap;
delete [] pMap;

-Howard

 

[Victor Bazarov]

hi there, i've got a problem:
can i create a dynamic array like this?

pMap = new char[mapX][mapY];

gcc (my compiler) sais, it's wrong...
i know that dynamic memory is more often used in this way:

pMap = new char[mapX];

but i need a second element...
can somebody help?

One way is to do this:
[...]

Two FAQs are related to this thread, I believe: #5.5 and #16.15.

V

 

[red floyd]

hi there, i've got a problem:
can i create a dynamic array like this?

pMap = new char[mapX][mapY];

gcc (my compiler) sais, it's wrong...
i know that dynamic memory is more often used in this way:

pMap = new char[mapX];

but i need a second element...
can somebody help?

One way is to do this:

char** pMap = new char*[mapY]; // does char* need () around it?
for (int i = 0; i < mapY; ++i)
pMap = new char[mapX];

and later,

for (int i = 0; i < mapY; ++i)
delete [] pMap;
delete [] pMap;

-Howard

But your memory isn't contiguous.

I'd allocate th rows as a big block:

char *pMapData = new char[mapX * mapY]
char **pMap = new char[mapY];
for (int = 0 ; i < mapY; ++i)
pMap = pMapData + (i * mapX);

Then you only have 2 delete[] calls, and the memory for the 2D array is
contiguous.


Or possibly, you could use a pointer to an array.

 

[Howard]

One way is to do this:
[...]

Two FAQs are related to this thread, I believe: #5.5 and #16.15.

Okay, okay, I know... I answered a question that's already in the FAQ (and
didn't answer it as well or as completely either, I might add). But just to
get my own stab back at you, Victor, this is from #5.5:


Note #1: Please don't give them the location of the appropriate FAQ. E.g.,
don't say, "Look at FAQ [10.3]" or "Look in section [10]". It's the old
give-them-a-fish vs. teach-them-to-fish problem.


Neener neener! (Sorry, I just couldn't resist)

-Howard

 

[Howard]

can i create a dynamic array like this?

pMap = new char[mapX][mapY];

One way is to do this:

char** pMap = new char*[mapY]; // does char* need () around it?
for (int i = 0; i < mapY; ++i)
pMap = new char[mapX];

and later,

for (int i = 0; i < mapY; ++i)
delete [] pMap;
delete [] pMap;

-Howard

But your memory isn't contiguous.

Nope. But that wasn't a specified requirement. I was just giving one way
to do it, not neccessarily the best.

I'd allocate th rows as a big block:

char *pMapData = new char[mapX * mapY]
char **pMap = new char[mapY];
for (int = 0 ; i < mapY; ++i)
pMap = pMapData + (i * mapX);

Then you only have 2 delete[] calls, and the memory for the 2D array is
contiguous.

Yep, something like that's probably better.

Or possibly, you could use a pointer to an array.

Or a vector!

Anyway, the FAQ explains the whole thing quite well. And, for the OP,
that's available at:

http://www.parashift.com/c++-faq-lite/

-Howard

 

[Mike Wahler]

hi there, i've got a problem:
can i create a dynamic array like this?

pMap = new char[mapX][mapY];

[snip]

I'd allocate th rows as a big block:

char *pMapData = new char[mapX * mapY]
char **pMap = new char[mapY];
for (int = 0 ; i < mapY; ++i)
pMap = pMapData + (i * mapX);

Then you only have 2 delete[] calls, and the memory for the 2D array is
contiguous.


Or possibly, you could use a pointer to an array.

I think better than what everyone else advised so far,
would be to use vector<vector<char> > or perhaps
vector<string>

Don't muck around with raw pointers unless you must.

$.02,
-Mike

 

[axter]

Check out the following link for good examples and multiple method for
creating a dynamic 2D array.

http://www.tek-tips.com/faqs.cfm?fid=5575