Question

In relation to question “reference to abstract class” I wrote the following example:

#include <iostream>
#include <vector>

class data
{
  public:
    virtual int get_value() = 0;
};

class data_impl : public data
{
  public:
    data_impl( int value_ ) : value( value_ ) {}
    virtual int get_value() { return value; }
  private:
    int value;
};

class database
{
  public:
    data& get( int index ) { return *v[index]; }
    void  add( data* d ) { v.push_back( d ); }
  private:
    std::vector< data* > v;
};

int main()
{
    data_impl d1( 3 );
    data_impl d2( 7 );

    database db;
    db.add( &d1 );
    db.add( &d2 );

    data& d = db.get( 0 );
    std::cout << d.get_value() << std::endl;
    d = db.get( 1 );
    std::cout << d.get_value() << std::endl;

    data& d_ = db.get( 1 );
    std::cout << d_.get_value() << std::endl;
    d_ = db.get( 0 );
    std::cout << d_.get_value() << std::endl;

    return 0;
}

To my surprise, the example prints:

3
3
7
7

and it looks like the reference assignment does the work differently from my expectation. I would expect:

3
7
7
3

Could you please point what my mistake is?

Thank you!

Answer 1

In

data& d = db.get( 0 );
std::cout << d.get_value() << std::endl;
d = db.get( 1 );
std::cout << d.get_value() << std::endl;

the first statement is a reference initialization, while the third statement is a slicing assignment.

You can not re-seat a reference.

Cheers & hth.,