return object reference

[zhouj]

what's the difference between the following two snippets of code? In a. B is returned; in b. B& is returned.
a.
class B
{
public: int i;
};

class D
{
public:
B fd(){B db; db.i=2; return db;}
};

b.
class B
{
public: int i;
};

class D
{
public:
B& fd(){B db; db.i=2; return db;}
};

[sarehu]

In the first version, a value of type B is returned.

And as you can see, in the second version, a value of type B& is returned.

[Ancient Dragon]

The second version is actually illegal because the object being returned is created on the stack, and that object is destroyed when the function returns, therefore it will not exist and the returned reference will be invalidated.

The first version (without reference) is ok because the program will actually duplicate the class when it is returned.

[zhouj]

Actually if you run the code on VC++2005, both will return 2.
#include <iostream> // Must #include this to use "placement new"
#include <string>
using namespace std;
1.
class B
{public: int i;};

class D
{
public:
B& fd(){B db; db.i=2; return db;}
};

int main()
{B b; D d; b=d.fd(); cout<<b.i<<endl;}

2.
#include <iostream> // Must #include this to use "placement new"
#include <string>
using namespace std;

class B
{public: int i;};

class D
{
public:
B fd(){B db; db.i=2; return db;}
};

int main()
{B b; D d; b=d.fd(); cout<<b.i<<endl;}

[Ancient Dragon]

>>{B b; D d; b=d.fd(); cout<<b.i<<endl;}
In program 1 that only works because it isn't using the reference that was returned but its using another copy of that class. Try this and retest: {D d; B& b=d.fd(); cout<<b.i<<endl;}