DirectX: Passing a array pointer to vertex structures

Question

I noticed some strangeness when passing a VERTEX structure to a buffer. At first I thought it was a faulty custom function for reading into .obj files, but debugging further narrowed the problem. Take for instance:

struct VERTEX{ 
    XMFLOAT3 Translation; 
    D3DXCOLOR Color;
};

and it's instance:

VERTEX foo[] = {
        {XMFLOAT3(-0.5f, 0.0f, 0.0f), D3DXCOLOR(1.0f,1.0f,1.0f,1.0f)}, 
        {XMFLOAT3(-0.5f, 0.5f, 0.0f), D3DXCOLOR(1.0f,1.0f,1.0f,1.0f)},
        {XMFLOAT3(0.0f, 0.0f, 0.0f), D3DXCOLOR(1.0f,1.0f,1.0f,1.0f)}, 
        {XMFLOAT3(0.0f, 0.5f, 0.0f), D3DXCOLOR(1.0f,1.0f,1.0f,1.0f)},
        {XMFLOAT3(0.9f, 0.0f, 0.0f), D3DXCOLOR(0.0f,1.0f,1.0f,1.0f)},
        {XMFLOAT3(2.0f, 0.9f, 0.0f), D3DXCOLOR(0.0f,1.0f,0.0f,1.0f)},
     };

Work perfectly when passed into a vertex buffer. Dynamically creating an array instance of the structure yields strange results. I'll see one vertex show up, or none. Here's an example, imagine foo as being a function returning a [working] VERTEX* array of vertices:

VERTEX *bar= new VERTEX[barsize];
VERTEX foo[] = {
        {XMFLOAT3(-0.5f, 0.0f, 0.0f), D3DXCOLOR(1.0f,1.0f,1.0f,1.0f)}, 
        {XMFLOAT3(-0.5f, 0.5f, 0.0f), D3DXCOLOR(1.0f,1.0f,1.0f,1.0f)},
        {XMFLOAT3(0.0f, 0.0f, 0.0f), D3DXCOLOR(1.0f,1.0f,1.0f,1.0f)}, 
        {XMFLOAT3(0.0f, 0.5f, 0.0f), D3DXCOLOR(1.0f,1.0f,1.0f,1.0f)},
        {XMFLOAT3(0.9f, 0.0f, 0.0f), D3DXCOLOR(0.0f,1.0f,1.0f,1.0f)},
        {XMFLOAT3(2.0f, 0.9f, 0.0f), D3DXCOLOR(0.0f,1.0f,0.0f,1.0f)},
     };
bar=foo;

And later on...

memcpy(MappedResource.pData, &bar, sizeof(bar));

Am I doing something funny trying to access the data that foo points to, such that only the first set of coordinates get passed in?

*edit: When writing this post, foo and bar got mixed. This post now shows the correct example.

your code is very confused, what does foo do? you said it is a function, but it is an array in your code, and foo=bar will get an uninitialized value. — zdd
– zdd, Commented Jan 9, 2014 at 2:02
I just edited the post, sorry. I dulled down the problem a bit. The original code has a function that parses an obj file for vertices. It returns a VEXTEX* array(represented as VERTEX foo[]), which is copied over to bar. I noticed that a VERTEX array works fine, but when I allocate using new, I can access the elements in the new array, but D3D can't - it seems to just draw a single point. I believe it to be a pointer issue. — Rashid Ellis
– Rashid Ellis, Commented Jan 9, 2014 at 2:10

zdd · Accepted Answer · 2014-01-09 02:24:46Z

1

The problem is this line

bar=foo;

memcpy(MappedResource.pData, &bar, sizeof(bar));

bar is a pointer of VERTEX, not a array of VERTEX, sizeof(bar) == 4 on x86 machine, so you will not get all the element copied.

you can first count how many Vertex element you need, and then when using memcpy, use sizeof(VERTEX) * vertex_count.

edited Jan 9, 2014 at 2:24

answered Jan 9, 2014 at 2:06

zdd

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4 Comments

Rashid Ellis Over a year ago

How would I reference the array sizeof() being pointed to: memcpy(MappedResource.pData, &bar, sizeof(&*bar));?

Rashid Ellis Over a year ago

How can I get the sizeof(VERTEX) in memory through it's pointer VERTEX*

Rashid Ellis Over a year ago

Thanks. It was making me crazy. I found this too: stackoverflow.com/questions/492384/…

Rashid Ellis Over a year ago

You definitely helped my understanding of my "missing memory" a bit further.

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