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The __LINE__ macro defaults to "int". That then gets handed to the debugger.

The __LINE__ macro, like all other macros, is expanded during the preprocessing of the source code and is not handed to the debugger in any way.

Yes... And debuggers that implement line numbers, generally work by taking that information as part of the preprocessing stage. And the #line and __LINE__ macro/directive were implemented _for debuggers_ when originally created. They were made to be handed over to the debugger.

If you simply compile and run, the debugger won't have __LINE__, no. But it also won't have line numbers, at all. So you might have missed a bit of context to this discussion - how are line numbers implemented in a debugger that does so, without access to the source?

No, the debugger does not get involved in preprocessing. When you write "a = __LINE__;", it expands to "a = 10;" (or whatever number) and is compiled, and the debugger has no knowledge of it. Debugging information, including the mapping of positions in the code to positions in the source, is generated by the compiler and embedded directly into the generated binary or an external file, from which the debugger reads it.

The __LINE__ macro is passed to the debugger only if the program itself outputs its value, and the "debugger" is a human reading that output :)

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