Chapter 10. The C++ Preprocessor

In This Chapter

  • Including source files

  • Defining constants and macros

  • Enumerating alternatives to constants

  • Inserting compile time checks

  • Simplifying declarations via typedef

You only thought that all you had to learn was C++. It turns out that C++ includes a preprocessor that works on your source files before the "real C++ compiler" ever gets to see it. Unfortunately, the syntax of the preprocessor is completely different than that of C++ itself.

Before you despair, however, let me hasten to add that the preprocessor is very basic and the C++ '09 standard has added a number of features that make the preprocessor almost unnecessary. Nevertheless, if the conversation turns to C++ at your next Coffee Club meeting, you'll be expected to understand the preprocessor.

What Is a Preprocessor?

Up until now, you may have thought of the C++ compiler as munching on your source code and spitting out an executable program in one step, but that isn't quite true.

First, the preprocessor makes a pass through your program looking for preprocessor instructions. The output of this preprocessor step is an intermediate file that has all the preprocessor commands expanded. This intermediate file gets passed to the C++ compiler for processing. The output from the C++ compiler is an object file that contains the machine instruction equivalent to your C++ source code. During the final step, a separate program known as the linker combines a set of standard libraries with your object ...

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