Friend class and NULL object member functions

[KriS]

Hi,

I have two simple questions:

1) Do I have to write storage-class-specifier, when I want to declare a
class as friend:

class Bar {};
class Foo { friend ??? Bar; };

VC++8 doesn't complain about it, g++ 3.4.4 throws an error and I can't
find anything about it in c++'98 standart.


2)
#include <iostream>

class Foo {
public: void print() { std::cout << "smth" << std::endl; }
};

int main() {
Foo* f = NULL;
f->print();
}

Works fine under g++ 3.4.4 and vc++8. As I understand this is undefined
behavior?

 

[Noah Roberts]

Hi,

I have two simple questions:

1) Do I have to write storage-class-specifier, when I want to declare a
class as friend:

class Bar {};
class Foo { friend ??? Bar; };

VC++8 doesn't complain about it, g++ 3.4.4 throws an error and I can't
find anything about it in c++'98 standart.

11.4.2 of 2003 states that a "elaborated-type-specifier" is required.

2)
#include <iostream>

class Foo {
public: void print() { std::cout << "smth" << std::endl; }
};

int main() {
Foo* f = NULL;
f->print();
}

Works fine under g++ 3.4.4 and vc++8. As I understand this is undefined
behavior?

That is correct.

 

[andrew]

That is correct.

More specifically, it only "works fine" because Foo:rint() does not
reference any member variables. If it did, it would find that the
implicit this Foo* parameter would be NULL and cause a runtime memory
access error. Its probably better to make methods that do not use member
variables static to remove temptation.

Andrew

 

[Fei Liu]

Hi,

I have two simple questions:

1) Do I have to write storage-class-specifier, when I want to declare a
class as friend:

class Bar {};
class Foo { friend ??? Bar; };

VC++8 doesn't complain about it, g++ 3.4.4 throws an error and I can't
find anything about it in c++'98 standart.

you have to tell the compiler either it's a class or a method. In this
case, you clearly wanted to grant class Bar friend access. So use
'friend class Bar;'

 

[Noah Roberts]

More specifically, it only "works fine" because Foo:rint() does not
reference any member variables.

"works fine" is one possible outcome of undefined behavior. Because
not referencing a member variable doesn't blow up here and now doesn't
mean it won't later or elsewhere. Undefined is undefined.