Trying to create copy function in a vector class

Question

I am working on implementing a vector class but cannot figure out how to write a function to copy one vector into another.

template <class T> class Vec {

public:
//TYPEDEFS
    typedef T* iterator;
    typedef const T* const_iterator;
    typedef unsigned int size_type;

//CONSTRUCTOS, ASSIGNMENT OPERATOR, & DESTRUCTOR
    Vec() {this->create(); }
    Vec(size_type n, const T& t = T()) { this->create(n, t); }
    Vec(const Vec& v) { copy(v); }
    Vec& operator=(const Vec& v);
    ~Vec() { delete [] m_data; }

//MEMBER FUNCTIONS AND OTHER OPERATORS
    T& operator[] (size_type i) { return m_data[i]; }
    const T& operator[] (size_type i) const { return m_data[i]; }
    void push_back (const T& t);
    iterator erase(iterator p);
    void resize(size_type n, const T& fill_in_value = T());
    void clear() { delete [] m_data; create(); }
    bool empty() const { return m_size == 0; }
    size_type size() const { return m_size; }

//ITERATOR OPERATIONS
    iterator begin() { return m_data; }
    const_iterator begin() const { return m_data; }
    iterator end() { return m_data + m_size; }
    const_iterator end() const { return m_data + m_size; }

private:
//PRIVATE MEMBER FUNCTIONS
    void create();
    void create(size_type n, const T& val);
    void copy(const Vec<T>& v);

//REPRESENTATION
    T *m_data;      //point to first location inthe allocated array
    size_type m_size;   //number of elements stored in the vector
    size_type m_alloc;  //number of array locations allocated, m_size <= m_alloc
};

//create an empty vector (null pointers everywhere)
template <class T> void Vec<T>::create() {
    m_data = NULL;
    m_size = m_alloc = 0;   //no memory allocated yet
}

//create a vector with size n, each location having the given value
template <class T> void Vec<T>::create(size_type n, const T& val) {
    m_data = new T[n];
    m_size = m_alloc = n;
    for (T* p = m_data; p != m_data + m_size; ++p)
        *p = val;
}   

//assign one vector to another, avoiding duplicate copying
template <class T> Vec<T>& Vec<T>::operator=(const Vec<T>& v) {
    if (this != &v) {
        delete [] m_data;
        this -> copy(v);
    }
    return *this;
}

This is the first things I came up with:

template <class T> void Vec<T>::copy(const Vec<T>& v) {

     m_size = m_alloc = v.size();
     m_data = &v;

}

I got an error about incompatible types...OK, makes sense that they are incompatible. So I take out the 'const' and now it works.

template <class T> void Vec<T>::copy(Vec<T>& v) {

     m_size = m_alloc = v.size();
     m_data = &v[0];

}

I am guessing this isn't entirely correct or good form. I'm not sure. And now I get an error about the pointer being freed not having been allocated (but it compiles, runs, and copies the vector successfully at least now). So I would say I am not really understanding passing variables/arrays/vectors/things by reference, and also dynamic allocation of memory. My question is: how can I improve the copy function that I have written to either not compare two incompatible variables, or to successfully allocate the pointer dynamically to the new vector so that I don't get that malloc error?

Answer 1

You need to do deep-copy of the elements, not simply assign the pointer m_data:

// precondition: `m_data` is not allocated
template <class T> void Vec<T>::copy(const Vec<T>& v) {
    m_data = new T[v.size()];
    m_size = m_alloc = v.size();
    for (size_t ii = 0; ii < m_size; ++ii)
        m_data[ii] = v[ii];
}

Answer 2

For your copy-constructor to be safe it needs to fail when the copy can't be made.

Vec<T>::Vec(const Vec<T>& o):m_size(o.m_size), m_alloc(o.m_size), m_data(new T()){
    std::copy( o.m_data, o.m_data+o.m_size, m_data);
}

The copy-constructor should be able to replace any Vec<T>::copy member.

The assignment is easily handled by introducing a swap function. This is exception safe.

void Vec<T>::swap(Vec<T>& rhs){
   std::swap(m_data, rhs.m_data);
   std::swap(m_size, rhs.m_size);
   std::swap(m_capacity, rhs.m_capacity);
}

With the exception safe Copy & swap & idiom it becomes:

Vec<T>& Vec<T>::operator = (const Vec<T>& rhs){
   Vec<T> cpy=rhs;
   swap( this, cpy);
   return *this;
}