Question

Today i’ve see very strange thing while working with references.

Just one simple example:

#include <iostream>

struct Base {
  enum Type {
    FOO = 0,
    BAR = 1
  };
  virtual ~Base() {}
  virtual Type type() const = 0;
  int value_;
};

struct Foo : Base { 
    Foo() { value_ = 33; }
    virtual Type type() const { return FOO; }
};

struct Bar : Base { 
    Bar() { value_ = 44; }
    virtual Type type() const { return BAR; }
};

int main() {
    Foo foo;
    Bar bar;
    Base & b = foo;
    std::cout << b.type() << ", " << b.value_ << "\n";
    b = bar;
    std::cout << b.type() << ", " << b.value_ << "\n";
    return 0;
}

What did you think output would be? I was really surprised when see it:

0, 33
0, 44

Tested on VS 2010, mingw 4.6, gcc 4.3. So, may be known secret of this magic?

Ideone link example

Answer 1

References are like pointers in C++, with two important exceptions (aside from syntax):

• Their cannot be assigned to null
• They cannot be reassigned

So, when you call b = bar, you are not reassigning the reference; you are assigning the value of bar to the object referenced by b; in this case, you are assigning the value of bar to foo. So, in the second line, you will have a Foo object with a value_ of 44. Just what your output says.