C++ FAQ Celebrating Twenty-One Years of the C++ FAQ!!!
(Click here for a personal note from Marshall Cline.)
Section 29:
29.1 What is this "newbie section" all about?
29.2 Where do I start? Why do I feel so confused, so stupid?
29.3 Should I use void main() or int main()?
29.4 Should I use f(void) or f()?
29.5 What are the criteria for choosing between short / int / long data types?
29.6 What the heck is a const variable? Isn't that a contradiction in terms?
29.7 Why would I use a const variable / const identifier as opposed to #define?
29.8 Are you saying that the preprocessor is evil?
29.9 What is the "standard library"? What is included / excluded from it?
29.10 How should I lay out my code? When should I use spaces, tabs, and/or newlines in my code?
29.11 Is it okay if a lot of numbers appear in my code?
29.12 What's the point of the L, U and f suffixes on numeric literals?
29.13 I can understand the and (&&) and or (||) operators, but what's the purpose of the not (!) operator?
29.14 Is !(a < b) logically the same as a >= b?
29.15 What is this NaN thing?
29.16 Why is floating point so inaccurate? Why doesn't this print 0.43?
29.17 Why doesn't my floating-point comparison work?
29.18 Why is cos(x) != cos(y) even though x == y? (Or sine or tangent or log or just about any other floating point computation)
29.19 What is the type of an enumeration such as enum Color? Is it of type int?
29.20 If an enumeration type is distinct from any other type, what good is it? What can you do with it?
29.21 What other "newbie" guides are there for me?
[29.7] Why would I use a const variable / const identifier as opposed to #define?

const identifiers are often better than #define because:

  • they obey the language's scoping rules
  • you can see them in the debugger
  • you can take their address if you need to
  • you can pass them by const-reference if you need to
  • they don't create new "keywords" in your program.

In short, const identifiers act like they're part of the language because they are part of the language. The preprocessor can be thought of as a language layered on top of C++. You can imagine that the preprocessor runs as a separate pass through your code, which would mean your original source code would be seen only by the preprocessor, not by the C++ compiler itself. In other words, you can imagine the preprocessor sees your original source code and replaces all #define symbols with their values, then the C++ compiler proper sees the modified source code after the original symbols got replaced by the preprocessor.

There are cases where #define is needed, but you should generally avoid it when you have the choice. You should evaluate whether to use const vs. #define based on business value: time, money, risk. In other words, one size does not fit all. Most of the time you'll use const rather than #define for constants, but sometimes you'll use #define. But please remember to wash your hands afterwards.