"syntax error: identifier" instantiating another class within same namespace?

"syntax error: identifier" instantiating another class within same namespace?

Post by Stephen Co » Sat, 12 Mar 2005 03:34:52


I've got 2 classes in 2 seperate header files, but within the same
namespace. If I use a line like:


// This code is inside Class2's header file
Class1 *newitem = new Class1(param1, param2);


I get "syntax error: identifier" and "undeclared identifier". Since
they're in the same namespace, and even in the same project, do I need
to do anything special, like put an "#include <theotherfile.h>" in each
header or anything???
 
 
 

"syntax error: identifier" instantiating another class within same namespace?

Post by Carl Danie » Sat, 12 Mar 2005 04:36:18


Yes.

The compiler only every looks at one file at a time. If you're compiling
x.cpp, the compiler sees x.cpp and everything that it includes (directly or
indirectly) in the order that it's included. Anything that you reference
must have been declared (and possibly defined) before the point of
reference.

Generally speaking, if you have mutually dependent classes, you'll have to
"forward declare" one of the classes. Any decent C++ book will cover the
necessities.

-cd

 
 
 

"syntax error: identifier" instantiating another class within same namespace?

Post by Stephen Co » Sat, 12 Mar 2005 04:47:10


Thanks for the response!

What if each class references the other one (circular references). How
would you handle that?
 
 
 

"syntax error: identifier" instantiating another class within same namespace?

Post by Carl Danie » Sat, 12 Mar 2005 07:47:41


class X;

class Y
{
X* m_X;
};

class X
{
Y* m_y;
};

If you need member functions of the classes to access members of the "other
type", you'll need to define your class implementation outside the class
definition:

struct X
{
int m_i;
void useY();
};

struct Y
{
int m_j;
void useX();
};

void X::useY()
{
Y y;
y.m_j;
}

void Y::useX()
{
X x;
x.m_i;
}

A "forward declaration" (or "incomplete class declaration" in standardese)
allows you to refer to a class in contexts that don't require knowing the
size or interface of a class (such as declaring a pointer or reference
variable or parameter). In order to declare a variable of the class type,
or access any members of the class, a complete class is required.

Between forward declarations and out of line member definitions, you should
be able to handle any sensible mutual dependency.

-cd
 
 
 

"syntax error: identifier" instantiating another class within same namespace?

Post by Bob Milto » Sat, 12 Mar 2005 07:49:20

Stephen,
Say you have classes X and Y that reference each other. The two headers
would look something like:

X.H:
class Y ;

class X
{
.
.
.
Y *myYRef ;
.
.
} ;

and Y.H would look something like:

class X ;

class Y
{
.
.
.
X *myXRef ;
.
} ;

Then the implementations of X and Y need to include both header files. (And
users of either may also need both headers).
Bob Milton