Add dynamically methods to a class : using macro function into header file - c++

Question

Trying to avoid write a repeating code; I used macros to delare methods into a class.
I want to define a macro function into header file like :
cracker.h
#include<message.h>
class Cracker {
#define DECLARE_MSG(MSG_NAME, MSG_TYPE,ALREADY_DEFINED) \
{ \
if(!ALREADY_DEFINED)
{ \
virtual bool cbProcessMsg_##MSG_NAME (API::MSG_NAME##Msg const & msg); \
}\
#include "message.h"
#undef DECLARE_MSG
};
message.h
DECLARE_MSG(AddOrder , 'A', false);
DECLARE_MSG(DeleteOrder , 'B', true);
DECLARE_MSG(ReplaceOrder , 'C', false);
...
But this code does note compile because I have not the right to add if statement outside a function.
Is there any alternative to do that ?

Answer 1

There's a couple of items worth highlighting:
1.
Macros do not know scope; the only way you can remove it's definition is to undefine it.
2.
macros are pasted into the code as is; and then compiled. A macro isn't a function itself, it's a code generation tool. This means that even if 'ALREADY_DEFINED' is true, you'll get another definition.
3.
What you're trying to do is add a function into the class outside of the class definition. You just can't do this. Ever.
4.
Header guards will save you pain for medium to large projects; you should get into the habbit of using them early.
While not quite the same, what it looks like you want to do is look up what the factory pattern is.

Answer 2

Use BOOST_PP_IIF if you (can) use the boost-library:
#define DECLARE_MSG(MSG_NAME, MSG_TYPE,ALREADY_DEFINED) \
BOOST_PP_IIF(ALREADY_DEFINED,, \
virtual bool cbProcessMsg_##MSG_NAME (API::MSG_NAME##Msg const & msg); \
)
You should use 0 and 1 instead of false and true here.
But if you don't, you can 'branch' definitions by concating name and number to another macro:
// two definitions for PP_CONCAT to expand macros, etc...
#define PP__CONCAT(a, b) a ## b
#define PP_CONCAT(a, b) PP__CONCAT(a, b)
// to expand your macro you should pass it through another macro
#define PP_EXPAND(...) __VA_ARGS__
#define DECLARE_MESSAGE_0(MSG_NAME, MSG_TYPE) /** the def when ALREADY_DEFINED = 0 */
#define DECLARE_MESSAGE_1(MSG_NAME, MSG_TYPE) /** the other branch */
#define DECLARE_MESSAGE(MSG_NAME, MSG_TYPE, ALREADY_DEFINED) \
PP_EXPAND(PP_CONCAT(DECLARE_MESSAGE_, ALREADY_DEFINED)(MSG_NAME, MSG_TYPE))
When you have problems understanding the preprocessor, you should check out the output with the option -E / running the code with the command cpp(which stands for c-preprogressor and not cplusplus(->c++))
However there may be another way to do the same thing, with templates and inheritance...