cpp-uniqueptr/main.cpp

#include <iostream>
#include <cassert>

/// Holds unique ownership of a pointed to object and destroys it automatically
template<class T>
class UniquePtr {
public:
    /// Constructor without custom deleter - use default deleter which just calls 'delete'
    explicit UniquePtr(T *object) : object(object), deleter([](T *object) -> void { delete object; }) {
        assert(object != nullptr);
    }

    /// Constructor with custom deleter
    /// Note that the custom deleter may not get a valid object (it could be a nullptr due to resetting, moving, etc)
    UniquePtr(T *object, void (*deleter)(T *)) : object(object), deleter(deleter) {
        assert(object != nullptr);
        assert(deleter != nullptr);
    }

    /// Copy constructor
    /// Shouldn't be used, since the pointer wouldn't be unique otherwise
    UniquePtr(const UniquePtr &other) = delete;

    /// Move constructor
    UniquePtr(UniquePtr &&other) : object(other.object), deleter(other.deleter) {
        other.object = nullptr;
    }

    /// Copy assignment operator
    /// Shouldn't be used, since the pointer wouldn't be unique otherwise
    UniquePtr &operator=(const UniquePtr &) = delete;

    /// Move assignment operator
    UniquePtr &operator=(UniquePtr &&other) {
        Swap(other.object);
        deleter = other.deleter;

        // TODO: If Swap is implemented with std::swap instead of deleting the current object, this should not be done!
        //  However, that would mean that the old object can stay in memory unnecessarily long (until 'other' goes out of scope)
        other.object = nullptr;
    }

    /// Destructor
    virtual ~UniquePtr() {
        // TODO: deleter(object) is called twice if 'Reset()' was previously called and the UniquePtr goes out of scope.
        //  With the default deleter, this is fine since 'delete object' handles the case of the object being null.
        //  However, with a custom deleter this may not be intended?
        //  We'd have to introduce a new flag and an if statement here
        deleter(object);
    }

    T operator*() const {
        assert(object != nullptr);

        return *object;
    }

    operator bool() const {
        return object != nullptr;
    }

    T *operator->() const {
        assert(object != nullptr);

        return object;
    }

    /// Release ownership of the owned object and return the pointer to it
    T *Release() {
        T *returnObject = object;
        object = nullptr;

        return returnObject;
    }

    /// Reset the internal pointer to null, deleting the owned object
    void Reset() {
        deleter(object);

        object = nullptr;
    }

    /// Swaps the currently owned object with another
    void Swap(T *other) {
        // TODO: This isn't really a "swap" but more of a "replace" - a proper swap would be std::swap(object, other);
        //  Not sure which one should be implemented, since I find this version more practical...
        deleter(object);

        object = other;
    }

private:
    T *object;

    void (*deleter)(T *);
};

/// Simple object for testing constructor, destructor calls and potential memory leaks
class TestObject {
public:
    TestObject() {
        std::cout << "Constructor" << std::endl;
        instanceCount += 1;
    }

    TestObject(const TestObject &other) {
        std::cout << "Copy constructor" << std::endl;
    }

    ~TestObject() {
        std::cout << "Destructor" << std::endl;
        instanceCount -= 1;
    }

    static void testPrint() {
        std::cout << "testPrint" << std::endl;
    }

    static int getInstanceCount() {
        return instanceCount;
    }

private:
    static int instanceCount;
};

int TestObject::instanceCount = 0;


void customTestObjectDeleter(TestObject *object) {
    std::cout << "Custom deleter" << std::endl;

    delete object;
}

int main() {
    {
        std::cout << "Constructing first pointer" << std::endl;
        UniquePtr pointer = UniquePtr<TestObject>(new TestObject());

        std::cout << "Calling test print via pointer if it bools to true" << std::endl;
        if (pointer) {
            pointer->testPrint();
        }

        std::cout << "Swapping for new object" << std::endl;
        pointer.Swap(new TestObject());

        std::cout << "Move assigning new pointer" << std::endl;
        UniquePtr pointer2 = UniquePtr<TestObject>(std::move(pointer));

        std::cout << "Constructing pointer with custom deleter" << std::endl;
        UniquePtr pointer3 = UniquePtr<TestObject>(new TestObject(), customTestObjectDeleter);

        std::cout << "Resetting that pointer" << std::endl;
        pointer3.Reset();

        if (pointer3) {
            std::cout << "This shouldn't happen!" << std::endl;
        }

        std::cout << "Moving into a pointer which already has an object" << std::endl;
        UniquePtr pointer4 = UniquePtr(new TestObject());
        UniquePtr pointer5 = UniquePtr(new TestObject());
        pointer4 = std::move(pointer5);
    }

    std::cout << std::endl << "All pointers are now out of scope, all objects should be deleted!" << std::endl;
    std::cout << TestObject::getInstanceCount() << " instances left" << std::endl;

    return 0;
}