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Thinking in C++
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Value substitution

When programming in C, the preprocessor is liberally used to create macros and to substitute values. Because the preprocessor simply does text replacement and has no concept nor facility for type checking, preprocessor value substitution introduces subtle problems that can be avoided in C++ by using const values.

The typical use of the preprocessor to substitute values for names in C looks like this:

#define BUFSIZE 100

BUFSIZE is a name that only exists during preprocessing, therefore it doesn’t occupy storage and can be placed in a header file to provide a single value for all translation units that use it. It’s very important for code maintenance to use value substitution instead of so-called “magic numbers.” If you use magic numbers in your code, not only does the reader have no idea where the numbers come from or what they represent, but if you decide to change a value, you must perform hand editing, and you have no trail to follow to ensure you don’t miss one of your values (or accidentally change one you shouldn’t).

Most of the time, BUFSIZE will behave like an ordinary variable, but not all the time. In addition, there’s no type information. This can hide bugs that are very difficult to find. C++ uses const to eliminate these problems by bringing value substitution into the domain of the compiler. Now you can say

const int bufsize = 100;

You can use bufsize anyplace where the compiler must know the value at compile time. The compiler can use bufsize to perform constant folding, which means the compiler will reduce a complicated constant expression to a simple one by performing the necessary calculations at compile time. This is especially important in array definitions:

char buf[bufsize];

You can use const for all the built-in types (char, int, float, and double) and their variants (as well as class objects, as you’ll see later in this chapter). Because of subtle bugs that the preprocessor might introduce, you should always use const instead of #define value substitution.

Thinking in C++
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   Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire