default values and negative indexes when creating vector class - c++

Question

I decided to train myself in classes and build a vecofint class, that would resemble a vector container with several methods.
My program works great, but I have found that I do not entirely understand some principles.
1. Default values:
Here is my class:
class vecofint {
public:
vecofint() : vec(new int[size1]) { zeros(); }
~vecofint() { delete vec; }
vecofint(const int size) { set(size); vec = new int[size1]; zeros(); }
vecofint& operator = (const vecofint& other) { vec = other.vec; size1 = other.size1; vec1 = other.vec1; }
vecofint(const vecofint& other) : vec(other.vec), size1(other.size1), vec1(other.vec1) {}
int getin(const int a) { return vec[a]; }
int get() { return size1; }
void set(const int a) { size1 = a; }
void setin(const int a, const int val) { vec[a] = val; }
int* pushback(const int val);
int* pushfront(const int val);
int* it;
void resize();
void resize1();
void zeros() {int * i = vec; fill(i, i + size1, 0);}
void print() { copy(vec, vec + size1, ostream_iterator<int>(cout, "\n")); }
private:
int size1 = 32;
int* vec;
int* vec1;
};
As you can see I assign all values of my created vector to 0. I do this for 2 reasons:
a) I have found that
vector <int> a(10);
copy(a.begin(), a.end(), ostream_iterator<int>(cout, " "));
prints 0 0 0 0 0 0 0 0 0 0;
b) I need some default values to implement my pushback method:
int* vecofint::pushback(const int val) {
int k = 0;
int* it = vec;
while (*it!=0) {
if (k == size1) {
resize();
it = vec+size1/2;
}
++k;
++it;
}
vec[k] = val;
return vec;
}
And I use zeros. But I do not like that, because it means that I cannot use 0 as not-default values.
First question:
How can I basically avoid these assignments to zeros, but still have basic values to iterate through the vector? Or maybe there are other methods on how to iterate through newly created space?
2) Negative indexes
Suppose I am trying, by mistake, access a negative index.
Second question:
What my program should write in this case? Some error?
I have found that in real vector nothing happens, at least until a destructor is called.