beam = new (beams*) [numElements]

[rontgen]

Ok, so this is a really basic question (or at least I think so).

What I'm trying to do is to use the default constructor to create an array of type beams that is numElements long.

I'm using gcc-g++ version 4.1.2 and I'm getting the following error:

error: array bound forbidden after parenthesized type-id
note: try removing the parentheses around the type-id

I've learned that this syntax was allowed in gcc-g++ versions prior to 4.0, but was then disallowed (for what ever reason).

Following the hint from the compiler actually makes the code compile, but I get an error at runt time. I guess this is due to that the statement is enterpreted as something else when leaving out the parantheses and this is where I get confused.

How can I write an equivalent expression without using the parantheses?

Thanks in advance
Rickard

[Disch]

If you want to create some beams, then you want to new some beams. You do not want to new beams*

The proper way to do it is like so:

beams* beam = new beams[numElements];

[rontgen]

Yes, but wat I want is an array of beams, compare int*[numElements]

[Disch]

Yes. new beams[numElements]; gives you an array of beams.

new beams*[numElements]; would give you an array of pointers, not beams (you might do this if you want a 2D "nested new" array - though I don't recommend that approach)

[rontgen]

Ok, maybe that's what I want then!?
I'm not the author of the code, which makes it so much harder to debug it...

[Disch]

From your description, it does not sound like you want a 2D array.

A 1D array would look like this:

// creation
beams* beam = new beams[numElements];

// accessing a single element
beam[x].whatever();

// cleanup
delete[] beam;

A 2D array would look like this:

// creation
beams** beam = new beams*[height];
for(int i = 0; i < height; ++i)
  beam[i] = new beams[width];

// accessing a single element
beam[y][x].whatever();

// cleanup
for(int i = 0; i < height; ++i)
  delete[] beam[i];
delete[] beam;

[rontgen]

Ok, now the following code:

Beam* beams = new Beam[numBeams];

generates the following output during compilation

773: error: conflicting declaration âBeam* beamsâ
644: error: âbeamsâ has a previous declaration as âBeam** beamsâ

Line 644 reads
Beam **beams = NULL;

[Disch]

Okay, "beams" already was declared as a Beam** (pointer to a pointer), so it looks like they did want a 2D array.

It's really hard to know what they want without seeing the code though.