Question on a class has not declared in a C++ header file.

[xiaoxiaoyang]

Hi,

I have a variable, previously was a constant, and defined in a header
file, e.g., A.h, and then now I would like to change this variable as
can be specified from a parameter file, such as B.cpp. like below:

in A.h before change, I have

static int constant CONST_X = 100;

Now I would like to be specified in B.cpp, that can read the variable
value from a file.

in B.cpp, I have:
static int X = 300; // can read from a file, or change in B.cpp.

I achieve the modification, in A.h, I did the below modification:

static int const CONST_X = class B::X;

or static int const CONST_X = B::X;

But I got error of "B has not been declared" when compiling.

or class B;
static int const CONST_X = B::X.

But I got error of incomplete type "B" used in nested named specified.

I think a stupid method is to comment out the line "static int const
CONST_C=100" in the original A.h, and replaced everywhere in every
class which uses CONST_C with B::X in the code (and define X in B.cpp
as above), but I think this method is so inefficient, and they most be
some method that allow me to only replace the CONST_X by B::X in the
A.h only, like I was trying to do but unsucceeded yet.

Can someone tell me how to solve this problem?

Thank you very much for the help.

Xiaoxiao

[Sam]

xi**********@live.com writes:

I achieve the modification, in A.h, I did the below modification:

static int const CONST_X = class B::X;

or static int const CONST_X = B::X;

This is an invalid constant initializer. In this context, the value must be
a constant expression. "B::X" is not a constant expression, that is, its
value is not known at compile time.
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[xiaoxiaoyang]

On Aug 24, 4:03*pm, Sam <[e-mail]:

*application_pgp-signature_part
< 1KViewDownload

[e-mail] writes:

I achieve the modification, in A.h, I did the below modification:

static int const CONST_X = class B::X;

or static int const CONST_X = B::X;

This is an invalid constant initializer. In this context, the value must be
a constant expression. "B::X" is not a constant expression, that is, its
value is not known at compile time.

Thanks a lot for the kind help, Sam. If that is the case, I must take
out of the const in the definiation of
static int const CONST_X = B::X, i.e., change it as static int CONST_X
= B::X, is that right?
If I still want the CONST_X be a constant, because it is a constant
read from a file, can you suggest me what can I do? If I can't do
anything on it, that's fine too.

[xiaoxiaoyang]

On Aug 24, 4:03*pm, Sam <[e-mail]:

*application_pgp-signature_part
< 1KViewDownload

[e-mail] writes:

I achieve the modification, in A.h, I did the below modification:

static int const CONST_X = class B::X;

or static int const CONST_X = B::X;

This is an invalid constant initializer. In this context, the value must be
a constant expression. "B::X" is not a constant expression, that is, its
value is not known at compile time.

After removing the const, i still got errors.

I just changed the static int const CONST_X = B::X to
static int CONST_X = B::X, but I still got error of "B has not
declared". If i changed it to
static int CONST_X = class B:: X, but I still got error of "expected
primary-expression before "class"".
If I changed it as:
class B;
static int CONST_X = B::X, then i still got error of "incomplete type
'B' used in nested name specifier.

Can anyone suggest what I can do? Thanks.

[Sam]

xi**********@live.com writes:

On Aug 24, 4:03Â*pm, Sam <[e-mail]:

>Â*application_pgp-signature_part
< 1KViewDownload

[e-mail] writes:

I achieve the modification, in A.h, I did the below modification:

static int const CONST_X = class B::X;

or static int const CONST_X = B::X;

This is an invalid constant initializer. In this context, the value must be
a constant expression. "B::X" is not a constant expression, that is, its
value is not known at compile time.

Thanks a lot for the kind help, Sam. If that is the case, I must take
out of the const in the definiation of
static int const CONST_X = B::X, i.e., change it as static int CONST_X
= B::X, is that right?

No. Static initializers can only be initialized by constant expressions.
Whether or not they, themselves, are static, is a different matter.

If you need to initialize a static object with a non-constant expression,
the only way to do this is to make it a class object, that's initialized by
a constructor.

If I still want the CONST_X be a constant, because it is a constant
read from a file, can you suggest me what can I do? If I can't do
anything on it, that's fine too.

The simplest and most straightforward way to do this, would be, as I said,
by making it a class object:

/* header: */

class A {

public:

A();

int value;
};

extern const A a_value;

/* module */

const A a_value;

A::A()
{
/* Here, write the code to initialize the value member by reading it
from a file */
}
The rest of your application would read the constant a_value.value, that's
initialized by your constructor.

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