Preprocessor ## pasting operator - c++

Question

What I am trying to achieve is creation of these three classes as mentioned in the code but just trying to use pre-processer in handy so that these similar classes can be created and executed rather than writing individual codes for them:
#include <iostream>
#define MYMACRO(len,baselen)
using namespace std;
class myclass ## len
{
int MYVALUE ## baselen;
public:
myclass ## len ## ()
{
cout << endl;
cout << " For class" ## len ## "'s function 'myFunction" ## len ## "' the value is: " << MYVALUE ## baselen << endl;
}
};
int main()
{
MYMACRO(10,100)
//myclass10 ob1;
MYMACRO(20,200)
//myclass20 ob2;
MYMACRO(30,300)
//myclass30 ob3;
myclass10 ob1;
myclass20 ob2;
myclass30 ob3;
cout << endl;
return 0;
}
Now I don't know whether the it can be done & since I am getting this error. If yes then please someone solve the error and enlighten me if no then please give the reason for the same so I am also reassured that we are on same page! The error is:
[root#localhost C++PractiseCode]# g++ -o structAndPreprocessor structAndPreprocessor.cpp
structAndPreprocessor.cpp:5: error: invalid token
structAndPreprocessor.cpp:6: error: invalid function declaration
structAndPreprocessor.cpp:7: error: invalid token
structAndPreprocessor.cpp:9: error: invalid token
structAndPreprocessor.cpp:9: error: invalid token
structAndPreprocessor.cpp:12: error: invalid token
structAndPreprocessor.cpp:12: error: invalid token
structAndPreprocessor.cpp:12: error: invalid token
structAndPreprocessor.cpp:12: error: invalid token
structAndPreprocessor.cpp:12: error: invalid token
structAndPreprocessor.cpp: In function `int main()':
structAndPreprocessor.cpp:25: error: `myclass10' was not declared in this scope
structAndPreprocessor.cpp:25: error: expected `;' before "ob1"
structAndPreprocessor.cpp:26: error: `myclass20' was not declared in this scope
structAndPreprocessor.cpp:26: error: expected `;' before "ob2"
structAndPreprocessor.cpp:27: error: `myclass30' was not declared in this scope
structAndPreprocessor.cpp:27: error: expected `;' before "ob3"
[root#localhost C++PractiseCode]#

Answer 1

You need to use \ at each end of your line to define your macro (and likely remove the using statement from the macro)
using namespace std;
#define MYMACRO(len,baselen) \
class myclass ## len \
{ \
int MYVALUE ## baselen; \
(...snip...) \
}\
};
Note the absence of escape on last line
Most likely you're doing Cpp and using Macros is discouraged. You'd better use either templates or traditional dynamic code (dpeending on your needs). Compared to macros, template bring the additional type checking at compilation and provide much more readable error messages.

Answer 2

The macro solution you present is a solution I've used before, but I would look at approaching this differently. The macro solution is a unwieldy and difficult to maintain and debug for all but the most trivial code.
Have you thought about generating the code you need from a template? Using Cheetah or Mako to fill out a source template would be quite a bit cleaner, and you could drive the generation from a configuration file, so you don't have to hand-maintain your list of classes.
You'd have a myclass.tmpl template file that looks something like this :
#for len, baselen in enumerate(mylist_of_classes_i_want_to_generate)
class MyClass$len
{
int MYVALUE$baselen;
public:
MyClass$len()
{
cout << endl;
cout << " For class $len's function 'myFunction $len' the value is: " << MYVALUE$baselen << endl;
}
};
#end for
You'd then call cheetah to autogenerate the code at the start of your build flow prior to compilation.