After example of use of #line

[Charles L]

Can someone provide me of an example of the use of the #line preprocessor
directive? I am not 100% sure how you use it.

Charles L

 

[Ivan Vecerina]

Can someone provide me of an example of the use of the #line preprocessor
directive? I am not 100% sure how you use it.

This directive is primarily of interest to code generators.
Such as, for example, GNU flex which generates a .c file from
a source file that contains a mix of flex-specific directives
and fragments of source code.

By adding #line directives in the generated code, the tool
can ensure that compiler diagnostics/error messages, and
even line numbers used by the debugger to step through code,
will refer to the correct line of the user-edited source
(instead of stepping through the generated code).

For a concrete example, best might be to try to use one
such code generator. E.g: http://www.gnu.org/software/flex/


Greetings,
Ivan

 

[John Harrison]

Can someone provide me of an example of the use of the #line preprocessor
directive? I am not 100% sure how you use it.

Charles L

It very rare that you would want to use it. Normally it would be used by
programs which generate programs not by human programmers.

Nevertheless ...

#line 200
// now the compiler thinks its on line 200

#line 300 "whatsit.cpp"
// now the compiler thinks its on line 300 of a file called whatsit.cpp

This might affect the printing of error messages for instance.

john

 

[Jack Klein]

Can someone provide me of an example of the use of the #line preprocessor
directive? I am not 100% sure how you use it.

Charles L

As others have pointed out, it is often produced by code generation
tools that produce C or C++ code from some other sort of input, so
that compilation error messages will contain a reference to the user
written source file name and line.

It can also be useful in production code that logs errors to a file
for later examination.