Memory overwrite in my own Vector class - c++

Question

For my Algorithm course project we can't use STL stuff like std::vector and so I'm trying to implement my own version of it (with templates).
It seems it works but when I declare a Vector< Vector< int > >
the .push() method starts to overwrite memory.
More specifically, with this code:
Vector<Vector<int>> v(3);
cout << v[0].push(0) << "\n";
cout << v[0].push(55) << "\n";
cout << v[0].push(4) << "\n";
cout << v[1].push(12) << "\n";
cout << v[1].push(3) << "\n";
cout << v[2].push(1) << "\n";
The output is this (.push() returns the address of where the element is inserted):
0x561328b0bc20
0x561328b0bc24
0x561328b0bc28
0x561328b0bc20
0x561328b0bc24
0x561328b0bc20
Any suggestion of why this happens?
Here is the code for my Vector class:
#include <iostream>
#include <bits/stdc++.h>
using namespace std;
template<class T>
class Vector {
private:
size_t _size;
size_t _capacity;
char* _buffer; //char* for performance
void _realloc(size_t);
public:
Vector(size_t s=0, T def=T());
T* push(T);
T& operator[](int);
size_t size() { return _size; }
};
template<class T>
void Vector<T>:: _realloc(size_t ncap)
{
_capacity = ncap;
char* nbuf = _buffer;
_buffer = new char[_capacity];
for(size_t i=0; i<_size * sizeof(T); ++i)
_buffer[i] = nbuf[i];
delete[] nbuf;
}
/*
* s -> size
* def -> default value
*/
template<class T>
Vector<T>:: Vector(size_t s, T def) : _size(s)
{
_capacity = 32;
while(_capacity < _size)
_capacity *= 2;
_buffer = new char[_capacity * sizeof(T)];
for(size_t i=0; i<_size; ++i)
((T*)_buffer)[i] = def;
}
/*
* check capacity, reallocs if necessary and push the element
* then return the addres (used only for debug)
*/
template<class T>
T* Vector<T>:: push(T ele)
{
if(_capacity == _size)
_realloc(2 * _capacity);
((T*)_buffer)[_size++] = ele;
return &((T*)_buffer)[_size-1];
}
template<class T>
T& Vector<T>:: operator[](int i)
{
if(i<0 or i>=(int)_size) {
cerr << "Out of bounds!\n";
abort();
}else
return ((T*)_buffer)[i];
}
template<class T>
ostream& operator<<(ostream& out, Vector<T>& v)
{
out << "{";
if(v.size() > 0) {
out << v[0];
for(size_t i=1; i<v.size(); ++i)
out << ", " << v[i];
}
out << "}";
return out;
}
Thanks!
PS: I know it's not a good use of C++ :P

Answer 1

Your operator= implicitly defined does the wrong thing. You use it in your constructor.
So, follow the rule of 0/3/5: Implement copy/move construtors/assignment and destructors, as this is an owning memory-management type. (Non-resource management types should follow the rule of 0; copyable resource management types the rule of 5.)
See rule of three. Implement all of destructor/copy constructor/move constructor/copy assign/move assign, or none of them.
Don't copy the data byte-wise when you realloc. std::move the Ts from the source to the dest.
In the copy construct/assign you'll want to copy the source Ts, not the underlying bytes.