Why we can use ## like this?

[purebel]

Head File:

#define CLASS_ITEM(name,price)
class ITEM_##name
{
  public :
     ITEM_##name();
     ......
     virtual ~ITEM_##name(){};
  private:
     ......
}

CPP File:
ITEM_book::ITEM_book(Char* name, int price){};

Why we can use ## like this?
Any feedback will be appreciated.

Thanks.
~Purebel

[ropez]

Please don't post the same question more than once!

[guruplus]

@purebel

You defined a macro

Advanced Macro Tricks
In addition to simple substitution, the preprocessor can also perform a bit of extra work on macro arguments, such as turning them into strings or pasting them together.
Pasting Tokens
Each argument passed to a macro is a token, and sometimes it might be 
expedient to paste arguments together to form a new token. This could come 
in handy if you have a complicated structure and you'd like to debug your program by printing out different fields. Instead of writing out the whole 
structure each time, you might use a macro to pass in the field of the structure to print.

To paste tokens in a macro, use ## between the two things to paste together.

For instance

#define BUILD_FIELD(field) my_struct.inner_struct.union_a.##field

Now, when used with a particular field name, it will expand to something like

my_struct.inner_struct.union_a.field1

The tokens are literally pasted together.  

[purebel]

Thanks very much!
It's clear for me now.

[purebel]

Thanks very much!
It's clear for me now.