suprising behavior passing arguments as references

[dwaggoner]

Consider the following do-nothing program

class myclass { };

void foo(myclass &x)
{
}

void foobar(const myclass &x)
{
}

myclass create(void)
{
  myclass x;
  return x;
}

int main(void)
{
  foobar(create());
  foo(create());
}

It will not compile under gcc (4.6.3). I get the following error message:

test.cpp: In function ‘int main()’:
test.cpp:21:15: error: invalid initialization of non-const reference of type ‘myclass&’ from an rvalue of type ‘const myclass’
test.cpp:4:6: error: in passing argument 1 of ‘void foo(myclass&)’

It will compile with the call to foo() commented out. I do not understand why the const is allowing foobar to compile? Any ideas?

Thanks

[Athar]

The standard says that temporary objects (rvalues) can be bound to const references, but not to non-const references. The object returned by create() is a temporary object.

[KRAkatau]

This happens because an r-value can be bound to a const reference, but not to a non-const reference. And a temporary unnamed object returned by the create() function is an r-value.

[KRAkatau]

Sometimes it's funny when the answers come almost at the same time and are pretty much the same :)

[vlad from moscow]

By the way the code will be compiled successfully by MS VC++ 2010 if you will not switch off VC++ language extensions for the project.:)
I described peculiarities of the MS VC++ 2010 here

http://cpp.forum24.ru/?1-3-0-00000045-000-0-0-1343495846

Though the description is written in Russian.

[dwaggoner]

Thanks. Of course the standard is what it is, but do you have any idea of the philosophy behind this particular rule? What are they protecting against with this behavior?

[vlad from moscow]

Consider a simple example

int x = 2, y = 3;
( x + y ) = 10;

If temporary values would be allowed to bind to non-const references you could write the code above.

[dwaggoner]

Ah -- the light goes on. So, in MS VC++ 2010 the above is permissible. Interesting.

Thanks

[vlad from moscow]

Apart from own features MS VC++ 2010 has numerous bugs and it allows for example the following code

strcut A
{
   int x;
};

A f() { A a = {}; return a; }

int main()
{
   f().x = 10;
}