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

It may help to think of the relationship between a "directive" and being "given directions" (i.e. orders). "preprocessor directives" are directions to the preprocessor about changes it should make to the code before the later stages of compilation kick in.

But, what's the preprocessor? Well, its name reflects that it processes the source code before the "main" stages of compilation. It's simply there to process the textual source code, modifying it in various ways. The preprocessor doesn't even understand the tokens it operates on - it has no notion of types or variables, classes or functions - it's all just quoted- and/or parentheses- grouped, comma- and/or whitespace separated text to be manhandled. This extra process gives more flexibility in selecting, combining and even generating parts of the program.

EDIT addressing @SWEngineer's comment: Many people find it helpful to think of the preprocessor as a separate program that modifies the C++ program, then gives its output to the "real" C++ compiler (this is pretty much the way it used to be). When the preprocessor sees #include <iostream> it thinks "ahhha - this is something I understand, I'm going to take care of this and not just pass it through blindly to the C++ compiler". So, it searches a number of directories (some standard ones like /usr/include and wherever the compiler installed its own headers, as well as others specified using -I on the command line) looking for a file called "iostream". When it finds it, it then replaces the line in the input program saying "#include " with the complete contents of the file called "iostream", adding the result to the output. BUT, it then moves to the first line it read from the "iostream" file, looking for more directives that it understands.

So, the preprocessor is very simple. It can understand #include, #define, #if/#elif/#endif, #ifdef and $ifndef, #warning and #error, but not much else. It doesn't have a clue what an "int" is, a template, a class, or any of that "real" C++ stuff. It's more like some automated editor that cuts and pastes parts of files and code around, preparing the program that the C++ compiler proper will eventually see and process. The preprocessor is still very useful, because it knows how to find parts of the program in all those different directories (the next stage in compilation doesn't need to know anything about that), and it can remove code that might work on some other computer system but wouldn't be valid on the one in use. It can also allow the program to use short, concise macro statements that generate a lot of real C++ code, making the program more manageable.