StrVec Class Definition

Having sketched the implementation, we can now define our StrVec class:

// simplified implementation of the memory allocation strategy for a vector-like class
class StrVec {
public:
    StrVec(): // the allocator member is default initialized
      elements(nullptr), first_free(nullptr), cap(nullptr) { }
    StrVec(const StrVec&);            // copy constructor
    StrVec &operator=(const StrVec&); // copy assignment
    ~StrVec();                        // destructor
    void push_back(const std::string&);  // copy the element
    size_t size() const { return first_free - elements; }
    size_t capacity() const { return cap - elements; }
    std::string *begin() const { return elements; }
    std::string *end() const { return first_free; }
    // ...
private:
    static std::allocator<std::string> alloc; // allocates the elements
    void chk_n_alloc()     // used by functions that add elements to a StrVec
        { if (size() == capacity()) reallocate(); }
    // utilities used by the copy constructor, assignment operator, and destructor
    std::pair<std::string*, std::string*> alloc_n_copy
        (const std::string*, const std::string*);
    void free();             // destroy the elements and free the space
    void reallocate();       // get more space and copy the existing elements
    std::string *elements;   // pointer to the first element in the array
    std::string *first_free; // pointer to the first free element in the array
    std::string *cap;        // pointer to one past the end of the array
};
// alloc must be defined in the StrVec implmentation file
allocator<string> StrVec::alloc;

The class body defines several of its members:

• The default constructor (implicitly) default initializes alloc and (explicitly) initializes the pointers to nullptr, indicating that there are no elements.

• The size member returns the number of elements actually in use, which is equal to first_free - elements.

• The capacity member returns the number of elements that the StrVec can hold, which is equal to cap - elements.

• The chk_n_alloc causes the StrVec to be reallocated when there is no room to add another element, which happens when cap == first_free.

• The begin and end members return pointers to the first (i.e., elements) and one past the last constructed element (i.e., first_free), respectively.