Redefine(#define) reserved c++ key word

Question

Is it possible to redefine a c++ keyword using #define?

#ifdef int
#undef int 
#define int 2
#endif
int main(){
    //Do something with int
}

I can't see the output in this case but i want to understand what happens internally. The reason I don't have #define is that I found that it is possible to #define a reserved keyword if you don't use a standard header file. I also tried to do run the following code.

#include<iostream>
using namespace std;
#ifdef int
#undef int 
#endif
int main(){
    cout<<int;
}

But te above throws the error at cout line.

This question is similar: http://stackoverflow.com/questions/2726204/c-preprocessor-define-ing-a-keyword-is-it-standards-conforming — anio, Aug 23 '12 at 22:20
@mfontanini I was just trying to be more familiar with the #define and c++ in general. I don't have a usecase for that.. — Ashutosh, Aug 26 '12 at 07:39
Possible duplicate of [C++ preprocessor #define-ing a keyword. Is it standards conforming?](https://stackoverflow.com/questions/2726204/c-preprocessor-define-ing-a-keyword-is-it-standards-conforming) — phuclv, Oct 22 '19 at 10:06

score 2 · Accepted Answer · edited May 23 '17 at 11:59

Is it possible? Yes. Is it good style? Absolutely not.

The preprocessor is not aware of C/C++ keywords, it only knows about preprocessor tokens and just does strict text replacement.

Your example is resulting in an error because you're #undefing it. Once you undefine it, it reverts to its previous behavior.

The only valid use I know of for doing something like this is to work around a bug in an old compiler, and that compiler is no longer relevant these days.

score 1 · Answer 2 · answered Aug 23 '12 at 22:33

Technically it works but it probably won't do you much good. If you want to use the standard C++ library you are not allowed define any of the keywords or any of a set of other names according to 17.6.4.3.1 [macro.names] paragraph 2:

A translation unit shall not #define or #undef names lexically identical to keywords, to the identifiers listed in Table 3, or to the attribute-tokens described in 7.6.

Mike Seymour · Answer 3 · 2012-08-23T22:33:28.883

You can, but you shouldn't.

In your examples, int doesn't get redefined, since it's wrapped in #ifdef int. That means "only do this if there's already a preprocessor macro called int", and there isn't.

If you just wrote #define int 2, then all occurrences of int would be replaced by 2; but then your code wouldn't compile since 2 main() {cout<<2;} is nonsense.

#undef will not remove a keyword from the language; it only removes preprocessor macros previously defined using #define.

score 0 · Answer 4 · answered Dec 16 '15 at 15:29

If you're don't use the standard libraries you're allowed to do so. In fact the preprocessor shouldn't distinguish between reserved and non-reserved words.

However that's probably not why you run into problems. First of all your examples don't do what you probably think. The fault is that int is normally not a preprocessor defined macro. The #ifdef int directive will therefore skip the following lines up to the terminating #endif.

What this means is that your second example expands to:

// stuff from iostream and possibly other headers 

int main(){
    cout<<int;
}

the fault is that cout<<int; simply isn't allowed.

Redefine(#define) reserved c++ key word

4 Answers4

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