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"syntax error: identifier" instantiating another class within samenamespace?

P: n/a
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???
Nov 17 '05 #1
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4 Replies


P: n/a
Stephen Corey wrote:
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???


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
Nov 17 '05 #2

P: n/a
Carl Daniel [VC++ MVP] wrote:
Stephen Corey wrote:
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???

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


Thanks for the response!

What if each class references the other one (circular references). How
would you handle that?
Nov 17 '05 #3

P: n/a
Stephen Corey wrote:
Carl Daniel [VC++ MVP] wrote:
Stephen Corey wrote:
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???

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


Thanks for the response!

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


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

Nov 17 '05 #4

P: n/a
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
{
Nov 17 '05 #5

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