problematic recursive include with String and Exception

[Karl]

Hey everyone!

So I'm trying to write a library which mimics the Java API using C++
(http://sf.net/projects/jalp4cpp). I've got some stuff done, but I
can't get the following code to work properly:

class String;

class Exception
{
public:
Exception();
Exception(String s);
String getMessage() const;
private:
String string;
};

class String
{
public:
String();
String(std::string s);
char charAt() const throw(Exception);
public:
std::string string;
};

Needless to say, the above dies with:

Exception.hpp:15: error: field 'string' has incomplete type

But if I try to flip the definition of String and Exception above, I
get the same error with Exception (because of the throw declaration).

I tried playing around with forward declarations, and #ifndef
#include-type stuff, and I can't get this to work.

I guess more importantly, I'm also trying to figure out why this
DOESN'T work. I mean, the code makes sense to me, but I'm not a
compiler. ;)

Thanks, everyone!

[Karl]

Oops! The link was actually http://sourceforge.net/projects/jal4cpp,
heh. :)

[Ian Collins]

Karl wrote:

Hey everyone!

So I'm trying to write a library which mimics the Java API using C++
(http://sf.net/projects/jalp4cpp). I've got some stuff done, but I
can't get the following code to work properly:

class String;

class Exception
{
public:
Exception();
Exception(String s);
String getMessage() const;
private:
String string;
};

class String
{
public:
String();
String(std::string s);
char charAt() const throw(Exception);
public:
private?
std::string string;
};

Needless to say, the above dies with:

Exception.hpp:15: error: field 'string' has incomplete type

Swap them and add

class Exception;

before String.

Exception is using a String, so String must be fully defined before
Exception.

String has a throw clause (which may be more trouble than it's worth),
which doesn't require the exception type to be fully defined.

Ian Collins.

[Greg]

Karl wrote:

Hey everyone!

So I'm trying to write a library which mimics the Java API using C++
(http://sf.net/projects/jalp4cpp). I've got some stuff done, but I
can't get the following code to work properly:

class String;

class Exception
{
public:
Exception();
Exception(String s);
String getMessage() const;
private:
String string;
};

class String
{
public:
String();
String(std::string s);
char charAt() const throw(Exception);
public:
std::string string;
};

Needless to say, the above dies with:

Exception.hpp:15: error: field 'string' has incomplete type

But if I try to flip the definition of String and Exception above, I
get the same error with Exception (because of the throw declaration).

I tried playing around with forward declarations, and #ifndef
#include-type stuff, and I can't get this to work.

I guess more importantly, I'm also trying to figure out why this
DOESN'T work. I mean, the code makes sense to me, but I'm not a
compiler. ;)

Thanks, everyone!

There's no way to solve the problem as long as Exception contains a
String member and a String method names Exception in its exception
specification. A type used in an exception specification must be (or be
a pointer or reference to) a complete type.

Personally, I would just turn the exception specification into a
comment. Otherwise the program shuts down were the routine (or one of
its called routines) ever throw something other than an Exception -
which can be difficult to ensure in practice; and at any rate having
the app quit suddenly without explanation is usually not the reason for
declaring the exception specification in the first place. Nor is it all
that surprising why exception specifications, other than throw(), are
rarely used in C++.

Greg