problem_12.md

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Problem 12: I want a vector of posns

2017-10-04

struct Posn {
    int x, y;
    Posn(int x, int y): x{x}, y{y} {}
};

int main() {
    vector<Posn> v; // Won't compile, why not?
}

Take a look at Vector's constructor:

template<typename T> vector<T>::vector(): n{0}, cap{1}, theVector{new T[cap]} {}

T[cap] creates an array of T objects. Which T objects will be stored in the array?

• C++ always calls a constructor when creating an object.
• Which constructor gets called? The default constructor
• But Posn doesn't have one

Need to separate memory allocation (Object creation step 1) from initialization (steps 2-4)

Allocation: void *operator new(size_t)

• Allocates size_t bytes
• No initialization
• Returns void*

Note:

• In C, void* implicity converts to any pointer type
• In C++, the conversion requires a cast

Initialization: "Placement new"

• new (address) type
• Constructs a "type" object at "address"
• Does not allocate memory (memory should already be allocated at "address")

template<typename T> class vector {
    ...
    public:
        vector(): n{0}, cap{1}, theVector{static_cast<T*>(operator new(sizeof(T)))} {}
        vector(size_t n, T x = T{}): 
            n{n}, cap{n}, theVector{static_cast<T*>(operator new(n *sizeof(T)))} {
            
            for (size_t i = 0; i < n; ++i)
                new(theVector + i) T(x);
        }

        ...

        void push_back(T x) {
            increase_cap();
            new(theVector + (n++)) T(x);
        }

        void pop_back() {
            if (n) {
                theVector[n-1].~T() // Must explicitly invoke destructor
                --n;
            }
        }

        ~vector() {
            destroy_items();
            operator delete(theVector);
        }

        void destroy_items() {
            for (auto &x: *this)
                x.~T();
        }
};

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