about common variable

[Rex_chaos]

I have define a MACRO varible as a common variable that be refered by
several classes.

#ifndef COMMON_VAR
#define COMMON_VAR 1

....

#endif

Whenever I want to change the value of common_var, I only need to
redefine it at the beginning of the source code. However, I am going
to compile all the classes into a library. That is, my code must link
to the library before I can use the classes. I found that redefine
the COMMON_VAR will no longer work because the classes has already
been compiled into a static library. How can I overcome this problem?

 

[WW]

I have define a MACRO varible as a common variable that be refered by
several classes.

There is not such thing as a "marco variable". It is a macro.

#ifndef COMMON_VAR
#define COMMON_VAR 1

...

#endif

Whenever I want to change the value of common_var, I only need to
redefine it at the beginning of the source code. However, I am going
to compile all the classes into a library. That is, my code must link
to the library before I can use the classes. I found that redefine
the COMMON_VAR will no longer work because the classes has already
been compiled into a static library. How can I overcome this problem?

You will need to use something else, not a macro. What do you want to do
exactly?

 

[Victor Bazarov]

I have define a MACRO varible as a common variable that be refered by
several classes.

There is no such thing as "MACRO varible". It's a macro. Nothing
variable about it.

#ifndef COMMON_VAR
#define COMMON_VAR 1

...

#endif

Whenever I want to change the value of common_var, I only need to
redefine it at the beginning of the source code. However, I am going
to compile all the classes into a library. That is, my code must link
to the library before I can use the classes.

"Compile classes into a library" and "code must link to the library"
are two different concepts. They cannot be connected with "that is".

I found that redefine
the COMMON_VAR will no longer work because the classes has already
been compiled into a static library. How can I overcome this problem?

Drop a macro and use a variable. Make it 'extern' if you expect
the users of your library to be able to change its value.

Read about "extern" linkage specifier in your favourite C++ book.

Victor