c++ passing by const pointer or reference to const

Question

I am learning c++, recently i read a book which gives a suggestion that you should use reference to const when possible (if base object will not be changed).

I have a question that should you pass reference to const pointer instead of const pointer, when possible, as reference prevents copied. If not when should i use reference to const pointer.

e.g:

Node(..., Node *const next = nullptr);

OR

Node(..., Node* const& next = nullptr);

Answer 1

Since references essentially are pointers there is no performance gain in passing pointers by reference.

Answer 2

You would only ever use a reference to a pointer if your intention was to modify the pointer value. For example:

ErrorCode MakeSomeObjectForMe(Object *&ptr) {
    if (badPreCondition) {
        return SomeSpecificError;
    }
    ptr = new Object();
    return Succuess;
} 

// Use ptr outside the function.

Otherwise it is not a good idea because it may cost you in performance through double indirection. So you will likely never pass a const & to a pointer.

Answer 3

As one example, in concurrent programming passing a ref to const ptr to worker thread may be used as a communication means; the ptr might be actually not const.

mutex sync;
void task(int*const&);
int main(){
    int *paramptr=nullptr;
    thread woker{task,paramptr};
    sync.lock();
    paramptr=whatever;
    sync.unlock();
    worker.join();
    return 0;
}

Answer 4

Passing by const reference is good practice when passing by value (that causes copying of parameters) is a heavy operation.

For example when you are passing a class with some properties to a function it's better to pass it by const reference, In other hand if your are passing types like int or just a pointer it's better not to use references because then you loos performance due to de-reference process.