Nested Comments in C/C++

Question

This is an interview question:-

Write a C program which when compiled and run, prints out a message indicating whether the compiler that it is compiled with, allows /* */ comments to nest.

The solution to this problem is given below:-

Sol:- you can have an integer variable nest:

int nest = /*/*/0*/**/1;

if it supports nested comments then the answer is 1 else answer is 0.

How is this working? I don't understand the variable declaration.

Answer 1

If the compiler doesn't allow nesting, the first */ will terminate the opening of the multiline comment, meaning the 0 won't be commented out. Written with some spaces:

int nest = /*/*/ 0 * /**/ 1;

resulting in the code

int nest = 0 * 1; // -> 0

If it allows nesting, it will be

int nest = /*/*/0*/**/ 1;

resulting in

int nest = 1;

Answer 2

The short answer to "how is this working" is that:

int nest = /*/*/0*/**/1;

with nested comments becomes something like:

int nest = 
// /* (comment level 1) 
//    /*/ (comment level 2) 
//         0
//    */*
// */
1;

and without, the extra * makes it:

int nest = 
// /*/ (comment level 1) 
// */
        0
    *
// /*
// */
1;

or 0*1 .

Or, I think that's what's happening, but this question is pretty much a disaster. I entirely agree with Blagovest Buyukliev's comment.

Answer 3

int nest = /*/*/0*/**/1;

Nesting not allowed

If nesting is NOT allowed, the first comment's range is:

           vvvvv
int nest = /*/*/0*/**/1;

With that comment removed (gap left for readability - the C++ preprocessor substitutes a single space, not sure about C), the next comment seen is:

                  vvvv
int nest =      0*/**/1;

With that also removed:

int nest =      0*    1;

Nesting allowed

Below, the |+- line shows the scope of the outer comment, and vvvvvv indicates teh scope of the inner comment.

           +---------+
           |         |
           | vvvvvv  |
int nest = /*/*/0*/**/1;

With those comments removed:

int nest =            1;

Answer 4

If it supports nested comments, then you'll have (Stripping the comments):

int nest = 1;

If it does not, then you'll have (Stripping the comments):

int nest = 0 * 1;

Answer 5

That is a big bag of loathsome hurt. My guess is that the third / possibly cancels the second multi-line comment block, rendering the * after the zero a multiplication, hence:

/* */0 * /* */ 1 == 0 * 1 == 0 // ==> nested comments aren't supported.

Answer 6

If the compiler understands nested comments, it will just strip the /*/*/0*/**/ part and leave you with the int nest = 1 .

Otherwise, it will see int nest = 0*1 and 0 * 1 == 0 .