Removing a member based on a template parameter - c++

Question

I would like to design a vector class with small vector optimisation. It looks like:
template <typename T, int small_size = 0>
class Vector {
private:
T data_small_[small_size];
T* data_;
T* size_;
T* capacity_;
public:
...
}
Unfortunately, most of the time, the class will be used with small_size = 0. Is there a way to remove data_small_ for small_size = 0 without going into template specialisation and rewriting the whole code for the class?

Answer 1

You can use the empty base optimization here. You would have to change Vector to not refer to data_small_ directly. Instead, write use SmallData to implement the required member functions and handle the small_size = 0 case in the partial specialization SmallData<T, 0>. Below is an example with SmallData implementing a copy constructor and operator[] and Vector using them without worrying about small_size.
template <typename T, int small_size>
class SmallData
{
public:
SmallData(const SmallData& other)
{
for(size_t i = 0; i < small_size; i++)
data_small_[i] = other.data_small_[i];
}
T& operator[](int k){return data_small_[k];}
protected:
T data_small_[small_size];
};
template <typename T>
class SmallData<T, 0>
{
public:
T& operator[](int k){//throw some error}
};
template <typename T, int small_size = 0>
class Vector : public SmallData<T, small_size>
{
public:
Vector(const Vector& other) : SmallData<T, small_size>(other)
{
//rest of copy ctor either here on in the member init list above
}
T& operator[](int k)
{
if(k<small_size) return SmallData<T, small_size>::operator[](k);
else return data_[k];
}
};