c++ - Preprocessor directives

Question

When we see #include <iostream>, it is said to be a preprocessor directive.

#include ---> directive

And, I think:

<iostream> ---> preprocessor

But, what is meant by "preprocessor" and "directive"?

Answer 1

#include is the preprocessor directive, <iostream> is just an argument supplied in addition to this directive, which in this case happens to be a file name.

Some preprocessor directives take arguments, some don't, e.g.

#define FOO 1

#ifdef _NDEBUG
    ....
#else
    ....
#endif

#warning Untested code !

The common feature is that they all start with #.

In Olden Times the preprocessor was a separate tool which pre-processed source code before passing it to the compiler front-end, performing macro substitutions and including header files, etc. These days the pre-processor is usually an integral part of the compiler, but it essentially just does the same job.