Dynamic memory for n00bs

[fafner]

I'm trying to ge tto grips with dynamic memory allocation, and I'm a bit confused. What is the difference between this:

int c;
std::cin >> c;
int b[c];

and this?

int c;
std::cin >> c;
int * b = new int[c];
delete(b);

Fafner

[Galik]

In the first the compiler automatically gives the memory back to the system. In the second you have to do it yourself.

[Athar]

Both examples contain an error, by the way.
You must declare an automatic array with a size that is a constant known at compile-time.
The second example compiles, but is also wrong. You have to delete an array that was created with new[] using delete[].

[m4ster r0shi]

Another difference is that the first allocates memory on the stack while the second does it on the heap. Also, the first is not standard c++ code. Not all compilers support it.

[mtweeman]

In the first you should write for example:

int b[4];

or:

enum NEW_TYPE {ONE=1, TWO=2, THREE=3, FOUR=4};
int b[FOUR];

I don't know if any compiler supports what you wrote (I mean first part).

[fafner]

Thanks all! The thing is that they both compile and run exactly the same way, which is why I asked in the first place. I find this weird, now that I've read your comments, but it's true nevertheless.

Fafner

EDIT: Using gcc btw

[AngelHoof]

I've never even seen that first code snippet :P

[m4ster r0shi]

fafner wrote:

Using gcc btw

That's why the first example works.

http://gcc.gnu.org/onlinedocs/gcc-2.95.3/gcc_4.html#SEC75

[Athar]

It's "officially" allowed to do that in C since the C99 standard, so it is no longer an extension for C...
but for C++ it remains an extension and so it might not work with other compilers (most notably VC++, which is a popular compiler in Windows environments).

[fafner]

Ah, I see! Excellent, thanks everybody;)