Using malloc/realloc for array of classes/structs including std vector

Question

I have a question wrt malloc/realloc memory that will contain an array of class/struct (i tried both struct and class the issue remains) members that include std vectors. I

Answer 1

/* dynamically allocate memory */
Comp= (voxel*)malloc(10*sizeof(voxel));

Comp is now a pointer to uninitialized memory.

for(i=0;i<10;++i) Comp[i] = v0;

This attempts to call Comp[i].operator=(v0), but Comp[i] is not a valid, initialized object. In a simple test/debug case, we may get lucky but in practice we'll get garbage and the vector will either try to free/use an invalid pointer.

That doesn't just mean you have to calloc() the memory instead, you can't make assumptions about what values an initialized object expects to find.

/* dynamically re-allocate memory */
Comp2= (voxel*)malloc(sizeof(voxel));  
printf("realloc done\n");

Comp2 is now a pointer to a single voxel, and no "realloc" was done.

for(i=0;i<10;++i){
  Comp2 =(voxel*)realloc(&Comp2[0], (i+1)*sizeof(voxel));
  Comp2[i] = v0;
}  

This is just bizzare. It starts with Comp2 pointing to a single voxel. Then you for some reason take the address of the first element (&Comp2[0]) rather than just using the address of the first element (Comp2), and you reallocate it to the same size. You then copy-assign v0 into the uninitialized memory at the last-but-one position:

Comp2 = [...uninit...]

for (i  = 0
realloc(i + 1 == 1)

Comp2 = [...uninit...]
              ^-- v0

i++
realloc(i+1 == 2)

Comp2 = [.....v0.....][...uninit...]
                            ^--v0

Short: You can't use malloc or calloc or realloc with non-pod objects. You might get away with it occasionally, but you are basically pointing a loaded shotgun at your foot.

It also seems that I cannot necessarily set an initial size of a vector in a class/struct

You can easily set the default size of a vector in a class, C++11 required (-std=c++11 or greater for gnu/clang compilers, VS2013 or higher)

#include 
#include 

struct A {
    std::vector v = { 1, 2, 3 }; // default population
};

struct B {
    std::vector v;
    B() : v(4) {}
};

int main() {
    A a;
    B b;
    std::cout << a.v.size() << ", " << b.v.size() << "\n";
    std::cout << "\n";
    for (int v : a.v) { std::cout << v << "\n"; }
    std::cout << "\n";
    for (int v : b.v) { std::cout << v << "\n"; }
}

http://ideone.com/KA9fWB