CODEWITHSUNDEEP
c++newlinestandards
Viktor
Viktor

Reputation: 155

C++ Standards (newline ending of source files)

I am referring to: Why should text files end with a newline? One of the answers quotes the C89 standard. Which in brief dictates that a file must end with a new line, which is not immediately preceded by a backslash.

Does that apply to the most recent C++ standard? 

#include <iostream>
using namespace std;

int main()
{
  cout << "Hello World!" << endl;
  return 0;
}
//\

Is the above valid? (Assuming there is a newline after //\, which I've been unable to display)

Upvotes: 12

Views: 2746

Answers (2)

xmh0511
xmh0511

Reputation: 7369

As per [lex.phases] p2 and p3, your particular case is also ill-formed in c++ standard. [lex.phases] p2 says

Each sequence of a backslash character () immediately followed by zero or more whitespace characters other than new-line followed by a new-line character is deleted, splicing physical source lines to form logical source lines. Only the last backslash on any physical source line shall be eligible for being part of such a splice. Except for splices reverted in a raw string literal, if a splice results in a character sequence that matches the syntax of a universal-character-name, the behavior is undefined. A source file that is not empty and that does not end in a new-line character, or that ends in a splice, shall be processed as if an additional new-line character were appended to the file.

Since you said

Assuming there is a newline after //, which I've been unable to display

Hence, the last visible \ is eligible as a splice. So, the sequence consisted of \ and the new-line character is deleted. It means the last character in this source file is / but without being followed by a newline. // starts a comment according to [lex.comment] p1

The characters // start a comment, which terminates immediately before the next new-line character.

As per [lex.phases] p3

The source file is decomposed into preprocessing tokens ([lex.pptoken]) and sequences of whitespace characters (including comments). A source file shall not end in a partial preprocessing token or in a partial comment.

In your case, the characters // start a comment but have no new line to terminate it. Hence, it's a partial comment. The program is ill-formed.

Upvotes: 1

AlexD
AlexD

Reputation: 32616

The given code is legal in the case of C++, but not for C.

Indeed, the C (N1570) standard says:

Each instance of a backslash character (\) immediately followed by a new-line character is deleted, splicing physical source lines to form logical source lines. Only the last backslash on any physical source line shall be eligible for being part of such a splice. A source file that is not empty shall end in a new-line character, which shall not be immediately preceded by a backslash character before any such splicing takes place.

The C++ standard (N3797) formulates it a bit differently (emphasis mine):

Each instance of a backslash character (\) immediately followed by a new-line character is deleted, splicing physical source lines to form logical source lines. Only the last backslash on any physical source line shall be eligible for being part of such a splice. If, as a result, a character sequence that matches the syntax of a universal-character-name is produced, the behavior is undefined. A source file that is not empty and that does not end in a new-line character, or that ends in a new-line character immediately preceded by a backslash character before any such splicing takes place, shall be processed as if an additional new-line character were appended to the file.

Upvotes: 9

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