简易vector的实现

在这里我们实现了一个简易的vector，没有利用到 stl中的内存分配器，内存分配利用的是new进行分配。其余功能大致实现 1 #ifndef _NVECTOR_

#define _NVECTOR_
#include<cstddef>
#include <algorithm>
template<typename T>
class nvector{
public:
    typedef T value_type;
    typedef value_type* pointer;
    typedef value_type& reference;
    typedef value_type* iterator;
    typedef size_t size_type;
    typedef ptrdiff_t difference_type;
private:
    iterator start; //迭代器的起始位置
    iterator finish;
    iterator end_of_storage;
    void insert_aux(iterator position, const T& value);
public:
    iterator begin(){return start;}
    iterator end(){return finish;}
    size_type size(){return size_type(end()-begin());}
    size_type capacity(){return size_type(end_of_storage-begin());}
    bool empty(){return start==finish;}
    reference operator [](size_type index){return *(begin()+index);}
    nvector():start(0),finish(0),end_of_storage(0){}
    nvector(size_type n, const T& value){
        start = new T[n+1];
        for(int i=0;i<n;i++)
            start[i] = value;
        finish = start+n;
        end_of_storage = start+n;
    }
    explicit nvector(size_type n){start = new T[n+1];finish = start; end_of_storage = start+n;}
    ~nvector(){if(start!=NULL) delete[] start;finish=0;end_of_storage = 0;}
    reference front(){return *begin();}
    reference back(){return *(end()-1);}
    void push_back(const T& value){
        if(finish!=end_of_storage){
            *finish = value;
            ++finish;
        }else
            insert_aux(finish, value);
    }
    void pop_back(){
        if(!empty()){
　　　　　　　　　*finish->~T();
            --finish;
　　　　　　}
    }
    void clear(){
        finish = start;
    }
};

template<typename T>
void nvector<T>::insert_aux(iterator position,const T& value){
    if(finish!=end_of_storage){
        T x_copy = value;
        copy_backward(position,finish,finish+1);
　　　　　　 finish++;
        *position = x_copy;
    }else{
        const size_type old_size = size();
        const size_type new_size = old_size!=0?2*old_size:1;
        iterator new_start = new T[new_size];
        iterator new_finish = copy(start,position,new_start);
        T x_copy = value;
        *new_finish = x_copy;
        ++new_finish;
        new_finish = copy(position,finish,new_finish);
        if(start!=NULL)
            delete[] start;
        start = new_start;
        finish = new_finish;
        end_of_storage = start+new_size;
    }
}

#endif