为什么 c++ 预处理文件中仍然存在 #line 预处理器指令？

Question

I know that the job of a preprocessor is to handle all preprocessor directives of a c++ program and ready the code for compiling. But I have found #line directives are present in almost all preprocessed files. How compiler understand a preprocessor directive while compiling the code?

Answer 1

Why there is still #line preprocessor directive present in a c++ preprocessed file?

Because the compiler needs information about the lines where the code used to be before the pre-processor expanded the directives. That information is used for diagnostic messages that report the exact file, line and column where the error is located.

How compiler understand a preprocessor directive while compiling the code?

I'm not sure whether #line is technically a pre-processor directive or not. But the compiler understands it for the same reason it understands anything: The smart people who implemented the compiler made it work.