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## Line numbering

### Just which line number anyway?

There are three reasonable requirements a cpplib client might have for the line number of a token passed to it:

- The source line it was lexed on.
- The line it is output on. This can be different to the line it was lexed on if, for example, there are intervening escaped newlines or C-style comments. For example:

      foo /* A long
      comment */ bar \
      baz
      ⇒
      foo bar baz

- If the token results from a macro expansion, the line of the macro name, or possibly the line of the closing parenthesis in the case of function-like macro expansion.

The `cpp_token` structure contains `line` and `col` members. The lexer fills these in with the line and column of the first character of the token. Consequently, but maybe unexpectedly, a token from the replacement list of a macro expansion carries the location of the token within the `#define` directive, because cpplib expands a macro by returning pointers to the tokens in its replacement list. The current implementation of cpplib assigns tokens created from built-in macros and the ‘`#`’ and ‘`##`’ operators the location of the most recently lexed token. This is a because they are allocated from the lexer’s token runs, and because of the way the diagnostic routines infer the appropriate location to report.

The diagnostic routines in cpplib display the location of the most recently lexed token, unless they are passed a specific line and column to report. For diagnostics regarding tokens that arise from macro expansions, it might also be helpful for the user to see the original location in the macro definition that the token came from. Since that is exactly the information each token carries, such an enhancement could be made relatively easily in future.

The stand-alone preprocessor faces a similar problem when determining the correct line to output the token on: the position attached to a token is fairly useless if the token came from a macro expansion. All tokens on a logical line should be output on its first physical line, so the token’s reported location is also wrong if it is part of a physical line other than the first.

To solve these issues, cpplib provides a callback that is generated whenever it lexes a preprocessing token that starts a new logical line other than a directive. It passes this token (which may be a `CPP_EOF` token indicating the end of the translation unit) to the callback routine, which can then use the line and column of this token to produce correct output.

### Representation of line numbers

As mentioned above, cpplib stores with each token the line number that it was lexed on. In fact, this number is not the number of the line in the source file, but instead bears more resemblance to the number of the line in the translation unit.

The preprocessor maintains a monotonic increasing line count, which is incremented at every new line character (and also at the end of any buffer that does not end in a new line). Since a line number of zero is useful to indicate certain special states and conditions, this variable starts counting from one.

This variable therefore uniquely enumerates each line in the translation unit. With some simple infrastructure, it is straight forward to map from this to the original source file and line number pair, saving space whenever line number information needs to be saved. The code the implements this mapping lies in the files `line-map.cc` and `line-map.h`.

Command-line macros and assertions are implemented by pushing a buffer containing the right hand side of an equivalent `#define` or `#assert` directive. Some built-in macros are handled similarly. Since these are all processed before the first line of the main input file, it will typically have an assigned line closer to twenty than to one.

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