Extracting data from a file that was read into memory - c++

Question

I have a binary data file that contains 2d and 3d coordinates in such order:
uint32 numberOfUVvectors;
2Dvec uv[numberOfUVvectors];
uint32 numberOfPositionVectors;
3Dvec position[numberOfPositionVectors];
uint32 numberOfNormalVectors;
3Dvec normal[numberOfNormalVectors];
2Dvec and 3Dvec are structs composed from 2 and 3 floats respectively.
At first, I read all these values using the "usual" way:
in.read(reinterpret_cast<char *>(&num2d), sizeof(uint32));
2Dvectors.reserve(num2d); // It's for an std::vector<2DVec> 2Dvectors();
for (int i = 0; i < num2d; i++){
2Dvec 2Dvector;
in.read(reinterpret_cast<char *>(&2Dvector), sizeof(2DVec));
2Dvectors.push_back(2Dvector);
}
It worked fine, but it was painfully slow (there can be more than 200k entries in a file and with so many read calls, the hdd access became a bottleneck). I decided to read the entire file into a buffer at once:
in.seekg (0, in.end);
int length = in.tellg();
in.seekg (0, in.beg);
char * buffer = new char [length];
is.read (buffer,length);
The reading is way faster now, but here's the question: how to parse that char buffer back into integers and structs?

Answer 1

To answer your specific question:
unsigned char * pbuffer = (unsigned char *)buffer;
uint32 num2d = *((uint32 *)pbuffer);
pbuffer += sizeof(uint32);
if(num2d)
{
2Dvec * p2Dvec = (2Dvec *)pbuffer;
2Dvectors.assign(p2Dvec, p2Dvec + num2d);
pbuffer += (num2d * sizeof(2Dvec));
}
uint32 numpos = *((uint32 *)pbuffer);
pbuffer += sizeof(uint32);
if(numpos)
{
3Dvec * p3Dvec = (3Dvec *)pbuffer;
Pos3Dvectors.assign(p3Dvec, p3Dvec + numpos);
pbuffer += (numpos * sizeof(3Dvec));
}
uint32 numnorm = *((uint32 *)pbuffer);
pbuffer += sizeof(uint32);
if(numnorm)
{
3Dvec * p3Dvec = (3Dvec *)pbuffer;
Normal3Dvectors.assign(p3Dvec, p3Dvec + numnorm);
pbuffer += (numnorm * sizeof(3Dvec));
}
// do not forget to release the allocated buffer
A an even faster way would be:
in.read(reinterpret_cast<char *>(&num2d), sizeof(uint32));
if(num2d)
{
2Dvectors.resize(num2d);
2Dvec * p2Dvec = &2Dvectors[0];
in.read(reinterpret_cast<char *>(&p2Dvec), num2d * sizeof(2Dvec));
}
//repeat for position & normal vectors

Answer 2

Use memcpy with the appropriate sizes and start values
or cast the values (example):
#include <iostream>
void copy_array(void *a, void const *b, std::size_t size, int amount)
{
std::size_t bytes = size * amount;
for (int i = 0; i < bytes; ++i)
reinterpret_cast<char *>(a)[i] = static_cast<char const *>(b)[i];
}
int main()
{
int a[10], b[10] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
copy_array(a, b, sizeof(b[0]), 10);
for (int i = 0; i < 10; ++i)
std::cout << a[i] << ' ';
}