xiaoxiaoy...@live.com kirjutas:
On Aug 24, 4:17 pm, Tonni Tielens <tonnitiel...@gmail.com> wrote:
On Aug 24, 9:31 pm, xiaoxiaoy...@live.com wrote:
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.
Initializing a static variable with another static variable defined
in another file is bad practice anyway, since you cannot tell what
the order of initialization will be. Eg. CONST_X can be initialized
before B::X, meaning CONST_X will have an undefined value. This of
course, will not happen for you at first, but only when you show your
application to your customer.- Hide quoted text -
- Show quoted text -
But my customer asks me to allow them change the variable value, i.e.,
read from a file. My B class can read from that file and give that X
value. Previously, the code of const X was defined as const in A.h,
and now the customer wants to change, so that's why i tried this. By
the way, i am new in C++, and the code was written by someone else
before and I need to make the modification according to customer's
need.
You have to decide if you want CONST_X to be const or not. If it is
const, it can be used by specifying the C-style array sizes and as an
integer template argument, for example. If it is not const, then it
cannot be used this way. This is a hard constraint imposed by the
language so there are no sneak-arounds.
If you are sure you want to make this variable non-const, then you
probably want to make it extern, not static, in order to ensure that all
compilation units see the same value:
// foo.h
extern int CONST_X; // BTW, all-caps is not a good style for a variable
This requires a definition somewhere:
// foo.cpp
int CONST_X = 0;
By some strange reason you want the same variable in class B. In order
to guarantee that the value of the variable is the same, I would suggest
to use a reference in class B. So when one assigns to the reference,
also the global variable changes:
// B.h
class B {
public:
static int& X;
// ...
};
Static class members also require definition somewhere:
// B.cpp
int& B::X = CONST_X;
Now you can do:
B::X = 42;
std::vector<char> buffer(CONST_X); // buffer of 42 bytes
Anyway, all this stuff remains quite convoluted, especially referring to
the same value from two places, and having a global non-const variable
in the first place.
hth
Paavo- Hide quoted text -
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Thanks Sam and Paavo's help. I will try them and let you know whether