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

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

static const int var = 5; #define var 5 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.

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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 :)

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In C, specifically? In C the correct answer is: use #define (or, if appropriate, enum)

While it is beneficial to have the scoping and typing properties of a const object, in reality const objects in C (as opposed to C++) are not true constants and therefore are usually useless in most practical cases.

So, in C the choice should be determined by how you plan to use your constant. For example, you can't use a const int object as a case label (while a macro will work). You can't use a const int object as a bit-field width (while a macro will work). In C89/90 you can't use a const object to specify an array size (while a macro will work). Even in C99 you can't use a const object to specify an array size when you need a non-VLA array.

If this is important for you then it will determine your choice. Most of the time, you'll have no choice but to use #define in C. And don't forget another alternative, that produces true constants in C - enum.

In C++ const objects are true constants, so in C++ it is almost always better to prefer the const variant (no need for explicit static in C++ though).

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The difference between static const and #define is that the former uses the memory and the later does not use the memory for storage. Secondly, you cannot pass the address of an #define whereas you can pass the address of a static const. Actually it is depending on what circumstance we are under, we need to select one among these two. Both are at their best under different circumstances. Please don't assume that one is better than the other... :-)

If that would have been the case, Dennis Ritchie would have kept the best one alone... hahaha... :-)

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In C #define is much more popular. You can use those values for declaring array sizes for example:

#define MAXLEN 5

void foo(void) {
   int bar[MAXLEN];
}

ANSI C doesn't allow you to use static consts in this context as far as I know. In C++ you should avoid macros in these cases. You can write

const int maxlen = 5;

void foo() {
   int bar[maxlen];
}

and even leave out static because internal linkage is implied by const already [in C++ only].

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Another drawback of const in C is that you can't use the value in initializing another const.

static int const NUMBER_OF_FINGERS_PER_HAND = 5;
static int const NUMBER_OF_HANDS = 2;

// initializer element is not constant, this does not work.
static int const NUMBER_OF_FINGERS = NUMBER_OF_FINGERS_PER_HAND 
                                     * NUMBER_OF_HANDS;

Even this does not work with a const since the compiler does not see it as a constant:

static uint8_t const ARRAY_SIZE = 16;
static int8_t const lookup_table[ARRAY_SIZE] = {
    1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16}; // ARRAY_SIZE not a constant!

I'd be happy to use typed const in these cases, otherwise...

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If you can get away with it, static const has a lot of advantages. It obeys the normal scope principles, is visible in a debugger, and generally obeys the rules that variables obey.

However, at least in the original C standard, it isn't actually a constant. If you use #define var 5, you can write int foo[var]; as a declaration, but you can't do that (except as a compiler extension" with static const int var = 5;. This is not the case in C++, where the static const version can be used anywhere the #define version can, and I believe this is also the case with C99.

However, never name a #define constant with a lowercase name. It will override any possible use of that name until the end of the translation unit. Macro constants should be in what is effectively their own namespace, which is traditionally all capital letters, perhaps with a prefix.

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It is ALWAYS preferable to use const, instead of #define. That's because const is treated by the compiler and #define by the preprocessor. It is like #define itself is not part of the code (roughly speaking).

Example:

#define PI 3.1416

The symbolic name PI may never be seen by compilers; it may be removed by the preprocessor before the source code even gets to a compiler. As a result, the name PI may not get entered into the symbol table. This can be confusing if you get an error during compilation involving the use of the constant, because the error message may refer to 3.1416, not PI. If PI were defined in a header file you didn’t write, you’d have no idea where that 3.1416 came from.

This problem can also crop up in a symbolic debugger, because, again, the name you’re programming with may not be in the symbol table.

Solution:

const double PI = 3.1416; //or static const...

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#define var 5 will cause you trouble if you have things like mystruct.var.

For example,

struct mystruct {
    int var;
};

#define var 5

int main() {
    struct mystruct foo;
    foo.var = 1;
    return 0;
}

The preprocessor will replace it and the code won't compile. For this reason, traditional coding style suggest all constant #defines uses capital letters to avoid conflict.

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I wrote quick test program to demonstrate one difference:

#include <stdio.h>

enum {ENUM_DEFINED=16};
enum {ENUM_DEFINED=32};

#define DEFINED_DEFINED 16
#define DEFINED_DEFINED 32

int main(int argc, char *argv[]) {

   printf("%d, %d\n", DEFINED_DEFINED, ENUM_DEFINED);

   return(0);
}

This compiles with these errors and warnings:

main.c:6:7: error: redefinition of enumerator 'ENUM_DEFINED'
enum {ENUM_DEFINED=32};
      ^
main.c:5:7: note: previous definition is here
enum {ENUM_DEFINED=16};
      ^
main.c:9:9: warning: 'DEFINED_DEFINED' macro redefined [-Wmacro-redefined]
#define DEFINED_DEFINED 32
        ^
main.c:8:9: note: previous definition is here
#define DEFINED_DEFINED 16
        ^

Note that enum gives an error when define gives a warning.

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The definition

const int const_value = 5;

does not always define a constant value. Some compilers (for example tcc 0.9.26) just allocate memory identified with the name "const_value". Using the identifier "const_value" you can not modify this memory. But you still could modify the memory using another identifier:

const int const_value = 5;
int *mutable_value = (int*) &const_value;
*mutable_value = 3;
printf("%i", const_value); // The output may be 5 or 3, depending on the compiler.

This means the definition

#define CONST_VALUE 5

is the only way to define a constant value which can not be modified by any means.

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Although the question was about integers, it's worth noting that #define and enums are useless if you need a constant structure or string. These are both usually passed to functions as pointers. (With strings it's required; with structures it's much more efficient.)

As for integers, if you're in an embedded environment with very limited memory, you might need to worry about where the constant is stored and how accesses to it are compiled. The compiler might add two consts at run time, but add two #defines at compile time. A #define constant may be converted into one or more MOV [immediate] instructions, which means the constant is effectively stored in program memory. A const constant will be stored in the .const section in data memory. In systems with a Harvard architecture, there could be differences in performance and memory usage, although they'd likely be small. They might matter for hard-core optimization of inner loops.

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Don't think there's an answer for "which is always best" but, as Matthieu said

static const

is type safe. My biggest pet peeve with #define, though, is when debugging in Visual Studio you cannot watch the variable. It gives an error that the symbol cannot be found.

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Incidentally, an alternative to #define, which provides proper scoping but behaves like a "real" constant, is "enum". For example:

enum {number_ten = 10;}

In many cases, it's useful to define enumerated types and create variables of those types; if that is done, debuggers may be able to display variables according to their enumeration name.

One important caveat with doing that, however: in C++, enumerated types have limited compatibility with integers. For example, by default, one cannot perform arithmetic upon them. I find that to be a curious default behavior for enums; while it would have been nice to have a "strict enum" type, given the desire to have C++ generally compatible with C, I would think the default behavior of an "enum" type should be interchangeable with integers.

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A simple difference:

At pre-processing time, the constant is replaced with its value. So you could not apply the dereference operator to a define, but you can apply the dereference operator to a variable.

As you would suppose, define is faster that static const.

For example, having:

#define mymax 100

you can not do printf("address of constant is %p",&mymax);.

But having

const int mymax_var=100

you can do printf("address of constant is %p",&mymax_var);.

To be more clear, the define is replaced by its value at the pre-processing stage, so we do not have any variable stored in the program. We have just the code from the text segment of the program where the define was used.

However, for static const we have a variable that is allocated somewhere. For gcc, static const are allocated in the text segment of the program.

Above, I wanted to tell about the reference operator so replace dereference with reference.

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We looked at the produced assembler code on the MBF16X... Both variants result in the same code for arithmetic operations (ADD Immediate, for example).

So const int is preferred for the type check while #define is old style. Maybe it is compiler-specific. So check your produced assembler code.

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I am not sure if I am right but in my opinion calling #defined value is much faster than calling any other normally declared variable (or const value). It's because when program is running and it needs to use some normally declared variable it needs to jump to exact place in memory to get that variable.

In opposite when it use #defined value, the program don't need to jump to any allocated memory, it just takes the value. If #define myValue 7 and the program calling myValue, it behaves exactly the same as when it just calls 7.

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