Why should one bother with preprocessor directives?

Question

This question may seem rather basic, but coming from an engineering (non computer-science) background, I was unsure about what the snippets of '#'s were in some C++ code.

A quick search led me to the concise, well-explained cplusplus tutorial page on preprocessor directives.

But why bother with the concept of preprocessor directives at all? Is it not possible to write equivalent code that can assign values to constants, define subroutines/function/macros and handle errors?

I guess I ultimately want to know when it is good practice to use such preprocessor directives, and when it is not.

Answer 1

You use preprocessor directives when you need to do something outside of the scope of the actual application. For instance, you'll see preprocessing done to include or not include code based on the architecture the executable is being built for. For example:

#ifdef _WIN32 // _WIN32 is defined by Windows 32 compilers #include <windows.h> #else #include <unistd.h> #endif 

Preprocessor directives are also used to guard includes so that classes/functions etc. are not defined more than once.

#ifndef MY_CLASS_6C1147A1_BE88_46d8_A713_64973C9A2DB7_H_ #define MY_CLASS_6C1147A1_BE88_46d8_A713_64973C9A2DB7_H_     class MyClass {     ....     }; #endif 

Another use is for embedding versioning inside of code and libraries.

In the Makefile you have something along the lines of:

-D MY_APP_VERSION=4.5.1 

While in the code you have

cout << "My application name version: " << MY_APP_VERSION << endl; 

Answer 2

Answer 1: conditional code that has to vary depending on what sort of computer it works on.

Answer 2: enabling and disabling language extensions and compatibility features seen at compile time.

The preprocessor came from C, where there were many thing you could not express. Good C++ code finds less reasons to use it than C code did, but sadly it's not quite useless.