C++ shared ptr implementation

very simple version for a quick whiteboarding interview questions.

#include <iostream>
#include <memory>
template <typename T>
class SharedPtr
{
public:
    explicit SharedPtr(T* ptr)
    {
        mPtr = ptr;
        mCount = new int(1);
    }
    ~SharedPtr()
    {
        (*mCount)--;
        if (*mCount == 0) {
            delete mPtr;
            delete mCount;
        }
    }
    // copy constructor
    SharedPtr(const SharedPtr& rhs)
    {
        mPtr = rhs.mPtr;
        mCount = rhs.mCount;
        (*mCount)++;
    }
    // copy assignment op
    SharedPtr& operator=(const SharedPtr& rhs)
    {
        if (this != &rhs) {
            mPtr = rhs.mPtr;
            (*mCount)++;
        }
        return *this;
    }
    T& operator*()
    {
        return *mPtr;
    }
    T* operator->()
    {
        // The overload of operator -> must either return a raw pointer,
        // or return an object (by reference or by value)
        // for which operator -> is in turn overloaded.
        return mPtr;
    }
    // for logging
    friend std::ostream& operator<<(std::ostream& os, SharedPtr<T>& sp)
    {
        os << "value:" << *sp << std::endl;
        return os;
    }
private:
    int* mCount{nullptr};
    T* mPtr{nullptr};
};
int main()
{
    SharedPtr<int> p(new int(3));
    SharedPtr<int> q(p);
    SharedPtr<int> r = p;
    std::cout << *p << std::endl;
    std::cout << *q << std::endl;
    std::cout << *r << std::endl;
}