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.