Block comments /* */ surprising behaviour with strings like "/home/*"

Language Design
1

Commenting out a block of code leads to a syntax error where a slash-star inside a string constant is interpreted as start of an inner block comment, which is different from C++ and Java behavior.

So in C++ no problem with block comment including myPath:

int main() {
/*
std::string myPath = “/home/*”;
std::cout <<"myPath is: " << myPath << std::endl;
*/
std::cout << “Hello, World!” << std::endl;
return 0;
}

But Kotlin gives syntax error no closing brace:

fun main(args: Array) {
/*
val myPath = “/home/*”;
println("myPath: " +myPath);
*/
println(“Hello World!”)
}

1 Like
2

I can confirm it’s broken, both Idea highlighting and compiler give syntax error
A workaround can be

fun main(args: Array<String>) {
/*
val myPath = "/home/${'*'}";
println("myPath: " +myPath);
*/
println("Hello World!")
}

then the commented code works the same.

1 Like
3

OK thanks.

4

However, the behavior is correct – compiler doesn’t parse what’s inside the comment so it’s irrelevant whether it’s inside a string or not.

The reason why this happens is because kotlin (as opposed to java and c++) supports nested comments, which is useful because you can comment multiple functions which have javadocs. In java it’s impossible.

In kotlin this compiles:

/*
/**
docs
*/
fun foo() = "bar"
*/  <-- comment block ends here

in Java it doesnt:

/*
/**
docs
*/  <-- comment block ends here
fun foo() = "bar" <- not a comment
*/ <-- compile Error
5

Yes, the different behaviour is a “feature” of Kotlin’s nested block comment support. I just find it very surprising that star-slash sequences embedded in strings are interpreted as begin or end comment markers. Do Kotlin programmers really begin or end a comment inside string constants? Certainly the non-nested block comment behaviour of C++/Java is not preserved.

1 Like
6

In your example, a star-slash sequence wasn’t embedded in a string, it was inside a comment. I don’t think comments should be analysed by a compiler (aside from looking for comment block markers).

1 Like
7

So you’re saying C language got it wrong? :wink:
What is surprising to me is different behaviour with heuristically the “same” code. You are saying it’s not the same because code is what the compiler sees, I think, and you are agreeing with how the Kotlin compiler sees it.
So my take away is that Kotlin block comments are in no way C/C++ style block comments. Further they are risky to use and you are better off using single line comments. Although I noticed CLion helpfully highlights occurrences of /* */ in comment blocks!

8

With that I agree, both for C and kotlin. TBH I have never used block comments aside from javadocs. Select, Ctrl + / . With vim plugin it’s especially convenient since you can correct selection.

1 Like