Converting std::vector of Vertices stucts to float* - c++

Question

What would be the best way of converting std::vector of Vertices to float*? I have vtx as my original data, which contains two vertices with position, normal and uv and I have std::vector of vertices v with the same position, normal and uv. What I am trying to achieve is getting the same memory layout and data as vtx into vtx2 using std::vector v. I tried copying the memory from v to vtx2 using memcpy but when I print them they are ordered in different way.
#include <iostream>
#include <vector>
using namespace std;
struct Vector3
{
float x;
float y;
float z;
};
struct Vector2
{
float x;
float y;
};
struct Vertex
{
Vector3 position;
Vector3 normal;
Vector2 uv;
};
int main(int argc, char *argv[])
{
const int n = 16;
float* vtx = new float[n];
// Vertex 1
// Position
vtx[0] = 1.0f;
vtx[1] = 2.0f;
vtx[2] = 3.0f;
// Normal
vtx[3] = 0.1f;
vtx[4] = 0.2f;
vtx[5] = 0.3f;
// UV
vtx[6] = 0.0f;
vtx[7] = 1.0f;
vtx += 8;
// Vertex 2
// Position
vtx[0] = 4.0f;
vtx[1] = 5.0f;
vtx[2] = 6.0f;
// Normal
vtx[3] = 0.2f;
vtx[4] = 0.3f;
vtx[5] = 0.4f;
// UV
vtx[6] = 0.0f;
vtx[7] = 1.0f;
vtx += 8;
for (int i = n; i>0; i--)
{
cout << *(vtx + i * -1) << endl;
}
vector<Vertex> v;
Vertex vt;
// Vertex 1
// Position
Vector3 pos1 = {1.0, 2.0, 3.0};
vt.position = pos1;
// Normal
Vector3 normal1 = {0.1, 0.2, 0.3};
vt.position = normal1;
// UV
Vector2 uv1 = {0.0, 1.0};
vt.uv = uv1;
v.push_back(vt);
// Vertex 2
// Position
Vector3 pos2 = {4.0, 5.0, 6.0};
vt.position = pos2;
// Normal
Vector3 normal2 = {0.2, 0.3, 0.4};
vt.position = normal2;
// UV
Vector2 uv2 = {0.0, 1.0};
vt.uv = uv2;
v.push_back(vt);
float* vtx2 = new float[n];
memcpy(vtx2, &v[0], v.size() * sizeof(Vertex));
for (int i = n; i>0; i--)
{
cout << *(vtx2 + i * -1) << endl;
}
delete[] vtx;
delete[] vtx2;
return 0;
}

Answer 1

#include <cstring>
#include <iostream>
#include <vector>
#include <cstddef>
using namespace std;
struct Vector3
{
float x;
float y;
float z;
};
struct Vector2
{
float x;
float y;
};
struct Vertex
{
Vector3 position;
Vector3 normal;
Vector2 uv;
};
int main(int argc, char *argv[])
{
const int n = 16;
float* vtx1 = new float[n];
float* vtx = vtx1;
cout << offsetof(Vertex, normal) << " " << offsetof(Vertex, uv) << " " << sizeof(Vertex) << "\n";
// Vertex 1
// Position
vtx[0] = 1.0f;
vtx[1] = 2.0f;
vtx[2] = 3.0f;
// Normal
vtx[3] = 0.1f;
vtx[4] = 0.2f;
vtx[5] = 0.3f;
// UV
vtx[6] = 0.0f;
vtx[7] = 1.0f;
vtx += 8;
// Vertex 2
// Position
vtx[0] = 4.0f;
vtx[1] = 5.0f;
vtx[2] = 6.0f;
// Normal
vtx[3] = 0.2f;
vtx[4] = 0.3f;
vtx[5] = 0.4f;
// UV
vtx[6] = 0.0f;
vtx[7] = 1.0f;
vtx += 8;
for (int i = n; i>0; i--)
{
cout << *(vtx + i * -1) << endl;
}
cout << "\n";
vector<Vertex> v;
Vertex vt;
// Vertex 1
// Position
Vector3 pos1 = {1.0, 2.0, 3.0};
vt.position = pos1;
// Normal
Vector3 normal1 = {0.1, 0.2, 0.3};
vt.normal = normal1;
// UV
Vector2 uv1 = {0.0, 1.0};
vt.uv = uv1;
v.push_back(vt);
// Vertex 2
// Position
Vector3 pos2 = {4.0, 5.0, 6.0};
vt.position = pos2;
// Normal
Vector3 normal2 = {0.2, 0.3, 0.4};
vt.normal = normal2;
// UV
Vector2 uv2 = {0.0, 1.0};
vt.uv = uv2;
v.push_back(vt);
float* vtx2 = new float[n];
vtx = vtx2;
memcpy(vtx, &v[0], n*sizeof(float));
vtx += n;
for (int i = n; i>0; i--)
{
cout << *(vtx + i * -1) << endl;
}
delete[] vtx1;
delete[] vtx2;
return 0;
}
Here is some corrected code with .normal instead of .position, it doesn't delete random memory by deleting vtx and the second print loop is fixed to show the data in the array instead of the 16 bytes of memory preceding it. It also prints the struct size and offsets in the first line. If you don't get 12 24 32 as the first line, your compiler is padding the structs with empty space which is causing your problems. You can use struct Vertex __attribute__((packed)) to prevent this on GCC or clang. Other compilers have different ways of doing it.

Answer 2

There is a bug in your code:
vt.position = normal1
should read
vt.normal = normal1
And similarly for the second vertex in your vector. Upon fixing that you may find the output matches (it does for me), but it may depend on how your compiler is padding structs.
For example, forcing a different alignment on Vector3 using struct Vector3 {...} __attribute__ ((aligned (16))); will generate "corrupted" output.