Question

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Difference between pointer variable and reference variable in C++

I am reading about the book "Inside the C++ Object Model" by Stanley Lippman. What puzzles me is the difference between a "reference" of an object and a "pointer" to an object. I know that a reference must be initialized when declared, while a pointer could be left for later initialization. But I want to know the physical implementation difference between them.

Why should there be the "reference" mechanism; isn’t it overlapping the function of a pointer? Under what circumstance should we use reference other than pointer? Many thanks.

10:48 AM 11/20/2021

Reference is from the semantic perspective.

Pointer is from the implementation perspective.

It’s kind of like the relation between what and how.

Answer 1

Most references are implemented using a pointer variable i.e. a reference usually takes up one word of memory. However, a reference that is used purely locally can – and often is – eliminated by the optimizer. For example:

  struct S { int a, int b[100]; };  
  void do_something(const vector<S>& v)
  {
    for (int i=0; i<v.size(); ++i) {
        int*& p = v[i].b;
          for (int j=0; j<100; ++j) cout <<p[j];
  }

In this case, p needs not be stored in memory (maybe it just exists in a register, maybe it disappears into the instructions).