"static const" vs "#define" vs "enum"

Question

Which one is better to use among the below statements in C?

static const int var = 5; 

or

#define var 5 

or

enum { var = 5 }; 

Answer 1

It depends on what you need the value for. You (and everyone else so far) omitted the third alternative:

1. static const int var = 5;
2. #define var 5
3. enum { var = 5 };

Ignoring issues about the choice of name, then:

• If you need to pass a pointer around, you must use (1).
• Since (2) is apparently an option, you don't need to pass pointers around.
• Both (1) and (3) have a symbol in the debugger's symbol table - that makes debugging easier. It is more likely that (2) will not have a symbol, leaving you wondering what it is.
• (1) cannot be used as a dimension for arrays at global scope; both (2) and (3) can.
• (1) cannot be used as a dimension for static arrays at function scope; both (2) and (3) can.
• Under C99, all of these can be used for local arrays. Technically, using (1) would imply the use of a VLA (variable-length array), though the dimension referenced by 'var' would of course be fixed at size 5.
• (1) cannot be used in places like switch statements; both (2) and (3) can.
• (1) cannot be used to initialize static variables; both (2) and (3) can.
• (2) can change code that you didn't want changed because it is used by the preprocessor; both (1) and (3) will not have unexpected side-effects like that.
• You can detect whether (2) has been set in the preprocessor; neither (1) nor (3) allows that.

So, in most contexts, prefer the 'enum' over the alternatives. Otherwise, the first and last bullet points are likely to be the controlling factors — and you have to think harder if you need to satisfy both at once.

If you were asking about C++, then you'd use option (1) — the static const — every time.

Answer 2

Generally speaking:

static const 

Because it respects scope and is type-safe.

The only caveat I could see: if you want the variable to be possibly defined on the command line. There is still an alternative:

#ifdef VAR // Very bad name, not long enough, too general, etc..   static int const var = VAR; #else   static int const var = 5; // default value #endif 

Whenever possible, instead of macros / ellipsis, use a type-safe alternative.

If you really NEED to go with a macro (for example, you want __FILE__ or __LINE__), then you'd better name your macro VERY carefully: in its naming convention Boost recommends all upper-case, beginning by the name of the project (here BOOST_), while perusing the library you will notice this is (generally) followed by the name of the particular area (library) then with a meaningful name.

It generally makes for lengthy names :)