What does ## in a #define mean?

Question

What does this line mean? Especially, what does ## mean?

#define ANALYZE(variable, flag)     ((Something.##variable) & (flag))

Edit:

Answer 1

One very important part is that this token concatenation follows some very special rules:

e.g. IBM doc:

• Concatenation takes place before any macros in arguments are expanded.
• If the result of a concatenation is a valid macro name, it is available for further replacement even if it appears in a context in which it would not normally be available.
• If more than one ## operator and/or # operator appears in the replacement list of a macro definition, the order of evaluation of the operators is not defined.

Examples are also very self explaining

#define ArgArg(x, y)          x##y
#define ArgText(x)            x##TEXT
#define TextArg(x)            TEXT##x
#define TextText              TEXT##text
#define Jitter                1
#define bug                   2
#define Jitterbug             3

With output:

ArgArg(lady, bug)   "ladybug"
ArgText(con)    "conTEXT"
TextArg(book)   "TEXTbook"
TextText    "TEXTtext"
ArgArg(Jitter, bug)     3

Source is the IBM documentation. May vary with other compilers.

To your line:

It concatenates the variable attribute to the "Something." and adresses a variable which is logically anded which gives as result if Something.variable has a flag set.

So an example to my last comment and your question(compileable with g++):

// this one fails with a compiler error
// #define ANALYZE1(variable, flag)     ((Something.##variable) & (flag))
// this one will address Something.a (struct)
#define ANALYZE2(variable, flag)     ((Something.variable) & (flag))
// this one will be Somethinga (global)
#define ANALYZE3(variable, flag)     ((Something##variable) & (flag))
#include 
using namespace std;

struct something{
int a;
};

int Somethinga = 0;

int main()
{
something Something;
Something.a = 1;

if (ANALYZE2(a,1))
    cout << "Something.a is 1" << endl;
if (!ANALYZE3(a,1))
    cout << "Somethinga is 0" << endl;
        return 1;
};