pointer parameter passed by reference in a function c++

Question

I have a class template which has a member variable std::vector<T> buffer and a member function setData , among others.

template <class T>
void ArrayT<T>::setData(const std::vector<T> * & data_ptr) {    
    buffer.assign(data_ptr->begin(), data_ptr->end());
}

I want setData to perform a deep copy of the data passed as its argument. As you can see, the argument is of type * & , which if I understand well is a pointer parameter passed by reference.

When I write:

vector<float> vec(3, 1.0);
vector<float>* vec2 = &vec;
A obj();
obj.setData(vec2);

I get an error:

E0434 a reference of type "const std::vector<float, std::allocator<float>> *&" (not const-qualified) cannot be initialized with a value of type "std::vector<float, std::allocator<float>> *"

Could someone please explain how to fix this?

Answer 1

The input parameter of setData() is a reference to a non-const pointer to a const std::vector . But the std::vector object you are pointing at is not const .

There is no reason to use a pointer at all in this situation. Change setData() to take the std::vector by const reference instead:

template <class T>
void ArrayT<T>::setData(const std::vector<T> &data)
{
    buffer.assign(data.begin(), data.end());
    // or simply: buffer = data;
}

vector<float> vec(3, 1.0);
Array<float> obj;
obj.setData(vec);

That being said, if you want to keep the pointer, you need to drop the const :

template <class T>
void ArrayT<T>::setData(std::vector<T> * data_ptr)
{
    buffer.assign(data_ptr->begin(), data_ptr->end());
    // or simply: buffer = *data_ptr;
}

Or at least move the const after the * to make the pointer itself const rather than the object it is pointing at:

template <class T>
void ArrayT<T>::setData(std::vector<T> * const data_ptr)
{
    buffer.assign(data_ptr->begin(), data_ptr->end());
    // or simply: buffer = *data_ptr;
}

Either way, notice I removed the reference. There is no good reason to pass a pointer by reference when you are not modifying what the pointer is pointing at. Pass the pointer by value instead.