C ++宏和重载

Question

I am working with c++ Macros, trying to implement on certain pattern. I have the following 3 macros:

#define First Objct t; t
#define Second a() / b()
#define Third ;

I've overload the lambda-ops in format: Objct [ Objct ]

So when I have in main() a line like:

First [ Second ] Third

it works fine, as it's 'translated' into:

Objct t; t[a()/b()]

(note: a() and b() are dummy functions returning type object.)

The hard part, is that I also have to make it work without the lambdas.

First Second Third

which means

Objct t; t a() / b()

In that case, I got a semicolon missing from FIRST to SECOND . I'm trying to figure out, what changes could be made (probably) to FIRST macro so it can compile in both cases. I'm not sure if I managed to explain myself properly. Any ideas?

Answer 1

Your question in pretty strange. I would strongly advise you not to use such a weird construct in real code. But I've looked at your question as if it were a funny puzzle.

Potential solution

I think there is no way to make both statement compile by changing only First . But if you change Second to:

#define Second +0,a() / b()

it compiles in both cases, under the sole condition that operator + is defined for Objct in combination with an int . If you manage to implement this operator without side effect, it would even produce what you expect.

Live demo

Why does it work ?

This definition makes use of the coma operator, the only issue being that the coma operator requires two expressions. +0 solves the issue syntactically, as +0 alone is valid, and t+0 is valid with the above-mentionned requirement.

With such a definition, First Second Third is preprocessed as

 Objct t; t +0,a() / b() ;     // comma operator evaluates t+0  
                               // then a()/b()

And First [Second] Third would be preprocessed as

 Objct t; t [+0,a() / b()] ;  // comma operator makes +0 being evaluated
                              // and lost and [] is called with value of a()/b().