how to declare the template parameter class as friend class - c++

fatihwk wrote:
> template <class C>
> class Session
> {
> public:
>
> protected:
> friend class C; //the line has no effect , compiled with vc8 sp1, it
> will report no public construct error.
> // It will ok if I changed to friend class
> Connection ;
> Session(C & c):m_c(c)
> {
> }
>
> C &m_c;
> };
>
>
> class Connection
> {
> public:
> Connection()
> {
> m_session = Session<Connection>(*this);
> }
>
> private:
> Session<Connection> m_session;
> }
>
> //so , is there any other way to resolve it ?

You cannot make any type which depends on a template
parameter a friend of the relevant class.

You could make a private base "interface" class for
Session, and then from the Session code pass a reference
to SessionBase to the Connection object for its use.

-- James

"fatihwk" <fatihwk@gmail.com> wrote in message
news:1190012404.212235.89050@r29g2000hsg.googlegroups.com...
> template <class C>
> class Session
> {
> public:
>
> protected:
> friend class C; //the line has no effect , compiled with vc8 sp1, it
> will report no public construct error.
> // It will ok if I changed to friend class
> Connection ;
> Session(C & c):m_c(c)
> {
> }
>
> C &m_c;
> };
>
>
> class Connection
> {
> public:
> Connection()
> {
> m_session = Session<Connection>(*this);
> }
>
> private:
> Session<Connection> m_session;
> }
>
> //so , is there any other way to resolve it ?

Even if you remove the protected in class Session, it fails to compile. Try
this instead, even though I get warning messages

Connection(): m_session(*this)
{
}

"James Dennett" <jdennett@acm.org> wrote in message
news:WmqHi.161581$zz2.148101@newsfe12.phx...
> fatihwk wrote:
>> template <class C>
>> class Session
>> {
>> public:
>>
>> protected:
>> friend class C; //the line has no effect , compiled with vc8 sp1, it
>> will report no public construct error.
>> // It will ok if I changed to friend class
>> Connection ;
>> Session(C & c):m_c(c)
>> {
>> }
>>
>> C &m_c;
>> };
>>
>>
>> class Connection
>> {
>> public:
>> Connection()
>> {
>> m_session = Session<Connection>(*this);
>> }
>>
>> private:
>> Session<Connection> m_session;
>> }
>>
>> //so , is there any other way to resolve it ?
>
> You cannot make any type which depends on a template
> parameter a friend of the relevant class.
>
> You could make a private base "interface" class for
> Session, and then from the Session code pass a reference
> to SessionBase to the Connection object for its use.

James, this compiles for me. I'm using MSVC++ .net 2003. Should it
compile?

template <class C>
class Session
{
public:

protected:
friend C;
Session(C & c):m_c(c)
{
}

C& m_c;
};


class Connection
{
public:
Connection(): m_session(*this)
{
}

private:
Session<Connection> m_session;
};

int main()
{
Connection Foo;
}

Jim Langston wrote:
> "James Dennett" <jdennett@acm.org> wrote in message
> news:WmqHi.161581$zz2.148101@newsfe12.phx...
>> fatihwk wrote:
>>> template <class C>
>>> class Session
>>> {
>>> public:
>>>
>>> protected:
>>> friend class C; //the line has no effect , compiled with vc8 sp1, it
>>> will report no public construct error.
>>> // It will ok if I changed to friend class
>>> Connection ;
>>> Session(C & c):m_c(c)
>>> {
>>> }
>>>
>>> C &m_c;
>>> };
>>>
>>>
>>> class Connection
>>> {
>>> public:
>>> Connection()
>>> {
>>> m_session = Session<Connection>(*this);
>>> }
>>>
>>> private:
>>> Session<Connection> m_session;
>>> }
>>>
>>> //so , is there any other way to resolve it ?
>> You cannot make any type which depends on a template
>> parameter a friend of the relevant class.
>>
>> You could make a private base "interface" class for
>> Session, and then from the Session code pass a reference
>> to SessionBase to the Connection object for its use.
>
> James, this compiles for me. I'm using MSVC++ .net 2003. Should it
> compile?
>
> template <class C>
> class Session
> {
> public:
>
> protected:
> friend C;
> Session(C & c):m_c(c)
> {
> }
>
> C& m_c;
> };
>
>
> class Connection
> {
> public:
> Connection(): m_session(*this)
> {
> }
>
> private:
> Session<Connection> m_session;
> };
>
> int main()
> {
> Connection Foo;
> }

No, it should not. (In C++03, you need "friend class Foo;"
to make a class a friend, and you can't make the class
named by a template a friend.)

g++ on my local machine diagnoses this with "error: using
template type parameter 'C' after 'class'" and confusingly
"error: friend declaration does not name a class or function".

-- James

this is a sample code, I just want to show the "friend" problem, so
there are something wrong.
Now I try to compile on different compilers,all are failed.
on g++ 3.4.4 cygwin, just one line error: test.cpp:7: error:
using typedef-name `C' after `class'

on g++ 3.3.3 suse linux :
testtemp.cpp:7: error: using template type parameter `C' after `class'
testtemp.cpp:7: error: ISO C++ forbids declaration of `type name' with
no type
testtemp.cpp: In constructor `Connection::Connection()':
testtemp.cpp:9: error: `Session<C>::Session(C&) [with C = Connection]'
is
protected
testtemp.cpp:21: error: within this context

so it means the class after friend must be a "real" class. not
typedef-name. so in the language what's the different between
original class and the typedefined class

this code compile ok on gcc 3.3.3 suse linux, but failed on cygwin
3.4.4.

class Connection;
template <class C >
class Session
{
public:
typedef Connection Conn;
protected:
friend class Conn; //here , we use the typedef-name
Session(C & c):m_c(c)
{
}

C &m_c;
};