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What is the difference between #define and const? [duplicate]
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Closed 9 years ago.
The macro #define MAX 80 is equivalent to const int MAX = 80; Both are constant and cannot be modified.
Isn't it better to use the macro instead of the constant integer? The constant integer takes memory. The macro's name is replaced by its value by the pre-processor, right? So it wouldn't take memory.
Why would I use const int rather than the macro?
Reason #1: Scoping. Macros totally ignore scope.
namespace SomeNS {
enum Functor {
MIN = 0
, AVG = 1
, MAX = 2
};
}
If the above code happens to be included in a file after the definition of the MAX macro, it will happily get preprocessed into 80 = 2, and fail compiling spectacularly.
Additionally, const variables are type safe, can be safely initialised with constant expressions (without need for parentheses) etc.
Also note that when the compiler has access to the const variable's definition when using it, it's allowed to "inline" its value. So if you never take its address, it does no even need not take up space.
There are a few reasons actually :
Scoping : you can't define a scope for a macro. It is present at global scope, period. Thus you can't have class-specific constants, you can't have private constants, etc. Also, you could end up with name collision, if you end up declaring something with the same name of a macro that you don't even know exists (in some lib/header you included f.e.)
Debugging : as the preprocessor just replaces instances of the macro with its value, it can become tricky to know why you got an error with a specific value (or just a specific behavior that you didn't expect...) . You have to remember where this value comes from. It is even more important in the case of reusable code, as you can even don't understand where does a value comes from, if it has been defined as a macro in a header you didn't write (thus it's not very good to do this yourself)
Adresses : a const variable is, well, a variable. It means notably that you can pass its adress around (when const pointers or const reference are needed), but you can't with macro
Type safety : you can specify a type for a const variable, something you can't for a macro.
As a general rule, I'd say that (in my opinion) you should avoid #define directives when you have a clear alternative (i.e. const variables, enums, inlines).
The thing is they aren't the same. The macro is just text substitution by the preprocessor while the const is a normal variable.
If someone ever tries to shadow MAX within a function (like const in MAX = 32;) they get a really weird error message when MAX is a macro.
In C++ the language-idiomatic approach is to use constants rather than macros. Trying to save a few bytes of memory (if it even saves them) doesn't seem worth the cost in readability.
1) Debugging is the main one for me. It's difficult for a debugger to resolve MAX to the value at run time, but it can do it with the const int version.
2) You don't get any type information with #define. If you're using a template-based function; say std::max where your other datum is a const int then the macro version will fail but the const int version will not. To work around that you'd have to use #define MAX 80U which is ugly.
3) You cannot control scoping with #define; it will apply to the whole compilation unit following the #define statement.
Related
In general in C++, you want to use constant instead of defining constants with #define as there is type checking and this is a good thing.
#define MYCONST 10; // NO
const int MYCONST = 10; // OK.
This is fine, but suppose I want to improve the performance of my app; if I have to read that constant still I might read it (I hope to be correct) from any cache level from L1 to L3 and this would introduce slowness.
Would it be better to define that constant as simple inline function like below?
inline int MYCONST()
{
return 10;
}
Am I correct when I should expect some improvement?
According to here for integer it seems that it depends on the compiler and the type I am using.
No and no: when you define something like
const int MYCONST = 10;
The value will not be read from "any cache level" but the compiler (at least any compiler build in the last 20 years) will issue exactly the same code as if you had used macros (or literals, which is equivalent), i.e. it will be placed directly inside the machine code.
Therefore your second suggestion (using an inline function) will not only have no performance benefit at all but prevent many uses of constants (like char my_array[MYCONST]), not to mention the lack of readability, wasted space etc. of your code.
Just follow the main C++ credo and use constants, there's nothing wrong with that :) ...
I think that defining a const is better practice anyway, but I also suspect that many compilers would not be able correctly to process a construct such as
char myBuffer[MYCONST()];
without issuing an error message.
My little C++ function needs to calculate a simple timeout value.
CalcTimeout(const mystruct st)
{
return (st.x + 100) * st.y + 200;
}
The numbers 100 and 200 would be confusing to read the code later so I would like to use #define for them. But these defines are only going to be needed for this function only, can I define them inside the function? The advantages this way are:
It is very local values and nobody else needs to know about it
Being closer to where it is used, the intent is clear, it has no other use, they are like local variables (except that they are not)
The disadvantage can be it is rather crude way to define something like local variable/const but it is obviously not local.
Other than that would this be odd to #define inside a C++ function? Most of the time we use #defines at the top of the file. Is using const variables better in any way in replacing a fixed local hard coded value like this?
The objective really is make code more readable/understandable.
Don't use a macro to define a constant; use a constant.
const int thingy = 100; // Obviously, you'll choose a better name
const int doodad = 200;
return (st.x + thingy) * st.y + doodad;
Like macros that expand to constant expressions, these can be treated as compile-time constants. Unlike macros, these are properly scoped within the function.
If you do have a reason for defining a macro that's only used locally, you can use #undef to get rid of it once you're done. But in general, you should avoid macros when (like here) there's a language-level construct that does what you want.
In C++ specifically it would be rather weird to see macros being used for that purpose. In C++ const completely replaces macros for defining manifest constants. And const works much better. (In C you'd have to stick with #define in many (or most) cases, but your question is tagged C++).
Having said that, pseudo-local macros sometimes come handy in C++ (especially in pre-C++11 versions of the language). If for some reason you have to #define such a macro "inside" a function, it is a very good idea to make an explicit #undef fro that macro at the end of the same scope. (I enclosed the word inside in quotes since preprocessor does not really care about scopes and can't tell "inside" from "outside".) That way you will be able to simulate the scoped visibility behavior other local identifiers have, instead of having a "locally" defined macro to spill out into the rest of the code all the way to the end of the translation unit.
Is it better to use static const vars than #define preprocessor? Or maybe it depends on the context?
What are advantages/disadvantages for each method?
Pros and cons between #defines, consts and (what you have forgot) enums, depending on usage:
enums:
only possible for integer values
properly scoped / identifier clash issues handled nicely, particularly in C++11 enum classes where the enumerations for enum class X are disambiguated by the scope X::
strongly typed, but to a big-enough signed-or-unsigned int size over which you have no control in C++03 (though you can specify a bit field into which they should be packed if the enum is a member of struct/class/union), while C++11 defaults to int but can be explicitly set by the programmer
can't take the address - there isn't one as the enumeration values are effectively substituted inline at the points of usage
stronger usage restraints (e.g. incrementing - template <typename T> void f(T t) { cout << ++t; } won't compile, though you can wrap an enum into a class with implicit constructor, casting operator and user-defined operators)
each constant's type taken from the enclosing enum, so template <typename T> void f(T) get a distinct instantiation when passed the same numeric value from different enums, all of which are distinct from any actual f(int) instantiation. Each function's object code could be identical (ignoring address offsets), but I wouldn't expect a compiler/linker to eliminate the unnecessary copies, though you could check your compiler/linker if you care.
even with typeof/decltype, can't expect numeric_limits to provide useful insight into the set of meaningful values and combinations (indeed, "legal" combinations aren't even notated in the source code, consider enum { A = 1, B = 2 } - is A|B "legal" from a program logic perspective?)
the enum's typename may appear in various places in RTTI, compiler messages etc. - possibly useful, possibly obfuscation
you can't use an enumeration without the translation unit actually seeing the value, which means enums in library APIs need the values exposed in the header, and make and other timestamp-based recompilation tools will trigger client recompilation when they're changed (bad!)
consts:
properly scoped / identifier clash issues handled nicely
strong, single, user-specified type
you might try to "type" a #define ala #define S std::string("abc"), but the constant avoids repeated construction of distinct temporaries at each point of use
One Definition Rule complications
can take address, create const references to them etc.
most similar to a non-const value, which minimises work and impact if switching between the two
value can be placed inside the implementation file, allowing a localised recompile and just client links to pick up the change
#defines:
"global" scope / more prone to conflicting usages, which can produce hard-to-resolve compilation issues and unexpected run-time results rather than sane error messages; mitigating this requires:
long, obscure and/or centrally coordinated identifiers, and access to them can't benefit from implicitly matching used/current/Koenig-looked-up namespace, namespace aliases etc.
while the trumping best-practice allows template parameter identifiers to be single-character uppercase letters (possibly followed by a number), other use of identifiers without lowercase letters is conventionally reserved for and expected of preprocessor defines (outside the OS and C/C++ library headers). This is important for enterprise scale preprocessor usage to remain manageable. 3rd party libraries can be expected to comply. Observing this implies migration of existing consts or enums to/from defines involves a change in capitalisation, and hence requires edits to client source code rather than a "simple" recompile. (Personally, I capitalise the first letter of enumerations but not consts, so I'd be hit migrating between those two too - maybe time to rethink that.)
more compile-time operations possible: string literal concatenation, stringification (taking size thereof), concatenation into identifiers
downside is that given #define X "x" and some client usage ala "pre" X "post", if you want or need to make X a runtime-changeable variable rather than a constant you force edits to client code (rather than just recompilation), whereas that transition is easier from a const char* or const std::string given they already force the user to incorporate concatenation operations (e.g. "pre" + X + "post" for string)
can't use sizeof directly on a defined numeric literal
untyped (GCC doesn't warn if compared to unsigned)
some compiler/linker/debugger chains may not present the identifier, so you'll be reduced to looking at "magic numbers" (strings, whatever...)
can't take the address
the substituted value need not be legal (or discrete) in the context where the #define is created, as it's evaluated at each point of use, so you can reference not-yet-declared objects, depend on "implementation" that needn't be pre-included, create "constants" such as { 1, 2 } that can be used to initialise arrays, or #define MICROSECONDS *1E-6 etc. (definitely not recommending this!)
some special things like __FILE__ and __LINE__ can be incorporated into the macro substitution
you can test for existence and value in #if statements for conditionally including code (more powerful than a post-preprocessing "if" as the code need not be compilable if not selected by the preprocessor), use #undef-ine, redefine etc.
substituted text has to be exposed:
in the translation unit it's used by, which means macros in libraries for client use must be in the header, so make and other timestamp-based recompilation tools will trigger client recompilation when they're changed (bad!)
or on the command line, where even more care is needed to make sure client code is recompiled (e.g. the Makefile or script supplying the definition should be listed as a dependency)
My personal opinion:
As a general rule, I use consts and consider them the most professional option for general usage (though the others have a simplicity appealing to this old lazy programmer).
Personally, I loathe the preprocessor, so I'd always go with const.
The main advantage to a #define is that it requires no memory to store in your program, as it is really just replacing some text with a literal value. It also has the advantage that it has no type, so it can be used for any integer value without generating warnings.
Advantages of "const"s are that they can be scoped, and they can be used in situations where a pointer to an object needs to be passed.
I don't know exactly what you are getting at with the "static" part though. If you are declaring globally, I'd put it in an anonymous namespace instead of using static. For example
namespace {
unsigned const seconds_per_minute = 60;
};
int main (int argc; char *argv[]) {
...
}
If this is a C++ question and it mentions #define as an alternative, then it is about "global" (i.e. file-scope) constants, not about class members. When it comes to such constants in C++ static const is redundant. In C++ const have internal linkage by default and there's no point in declaring them static. So it is really about const vs. #define.
And, finally, in C++ const is preferable. At least because such constants are typed and scoped. There are simply no reasons to prefer #define over const, aside from few exceptions.
String constants, BTW, are one example of such an exception. With #defined string constants one can use compile-time concatenation feature of C/C++ compilers, as in
#define OUT_NAME "output"
#define LOG_EXT ".log"
#define TEXT_EXT ".txt"
const char *const log_file_name = OUT_NAME LOG_EXT;
const char *const text_file_name = OUT_NAME TEXT_EXT;
P.S. Again, just in case, when someone mentions static const as an alternative to #define, it usually means that they are talking about C, not about C++. I wonder whether this question is tagged properly...
#define can lead to unexpected results:
#include <iostream>
#define x 500
#define y x + 5
int z = y * 2;
int main()
{
std::cout << "y is " << y;
std::cout << "\nz is " << z;
}
Outputs an incorrect result:
y is 505
z is 510
However, if you replace this with constants:
#include <iostream>
const int x = 500;
const int y = x + 5;
int z = y * 2;
int main()
{
std::cout << "y is " << y;
std::cout << "\nz is " << z;
}
It outputs the correct result:
y is 505
z is 1010
This is because #define simply replaces the text. Because doing this can seriously mess up order of operations, I would recommend using a constant variable instead.
Using a static const is like using any other const variables in your code. This means you can trace wherever the information comes from, as opposed to a #define that will simply be replaced in the code in the pre-compilation process.
You might want to take a look at the C++ FAQ Lite for this question:
http://www.parashift.com/c++-faq-lite/newbie.html#faq-29.7
A static const is typed (it has a type) and can be checked by the compiler for validity, redefinition etc.
a #define can be redifined undefined whatever.
Usually you should prefer static consts. It has no disadvantage. The prprocessor should mainly be used for conditional compilation (and sometimes for really dirty trics maybe).
Defining constants by using preprocessor directive #define is not recommended to apply not only in C++, but also in C. These constants will not have the type. Even in C was proposed to use const for constants.
Always prefer to use the language features over some additional tools like preprocessor.
ES.31: Don't use macros for constants or "functions"
Macros are a major source of bugs. Macros don't obey the usual scope
and type rules. Macros don't obey the usual rules for argument
passing. Macros ensure that the human reader sees something different
from what the compiler sees. Macros complicate tool building.
From C++ Core Guidelines
As a rather old and rusty C programmer who never quite made it fully to C++ because other things came along and is now hacking along getting to grips with Arduino my view is simple.
#define is a compiler pre processor directive and should be used as such, for conditional compilation etc.. E.g. where low level code needs to define some possible alternative data structures for portability to specif hardware. It can produce inconsistent results depending on the order your modules are compiled and linked. If you need something to be global in scope then define it properly as such.
const and (static const) should always be used to name static values or strings. They are typed and safe and the debugger can work fully with them.
enums have always confused me, so I have managed to avoid them.
Please see here: static const vs define
usually a const declaration (notice it doesn't need to be static) is the way to go
If you are defining a constant to be shared among all the instances of the class, use static const. If the constant is specific to each instance, just use const (but note that all constructors of the class must initialize this const member variable in the initialization list).
In embedded programming, for example, #define GLOBAL_CONSTANT 42 is preferred to const int GLOBAL_CONSTANT = 42; for the following reasons:
it does not need place in RAM (which is usually very limited in microcontrollers, and µC applications usually need a large number of global constants)
const needs not only a storage place in the flash, but the compiler generates extra code at the start of the program to copy it.
Against all these advantages of using #define, what are the major advantages of using const?
In a non-µC environment memory is usually not such a big issue, and const is useful because it can be used locally, but what about global constants? Or is the answer just "we should never ever ever use global constants"?
Edit:
The examples might have caused some misunderstanding, so I have to state that they are in C. If the C compiler generated the exact same code for the two, I think that would be an error, not an optimization.
I just extended the question to C++ without thinking much about it, in the hopes of getting new insights, but it was clear to me, that in an object-oriented environment there is very little space for global constants, regardless whether they are macros or consts.
Are you sure your compiler is too dumb to optimize your constant by inserting its value where it is needed instead of putting it into memory? Compilers usually are good in optimizations.
And the main advantage of constants versus macros is that constants have scope. Macros are substituted everywhere with no respect for scope or context. And it leads to really hard to understand compiler error messages.
Also debuggers are not aware of macros.
More can be found here
The answer to your question varies for C and C++.
In C, const int GLOBAL_CONSTANT is not a constant in C, So the primary way to define a true constant in C is by using #define.
In C++, One of the major advantage of using const over #define is that #defines don't respect scopes so there is no way to create a class scoped namespace. While const variables can be scoped in classes.
Apart from that there are other subtle advantages like:
Avoiding Weird magical numbers during compilation errors:
If you are using #define those are replaced by the pre-processor at time of precompilation So if you receive an error during compilation, it will be confusing because the error message wont refer the macro name but the value and it will appear a sudden value, and one would waste lot of time tracking it down in code.
Ease of Debugging:
Also for same reasons mentioned in #2, while debugging #define would provide no help really.
Another reason that hasn't been mentioned yet is that const variables allow the compiler to perform explicit type-checking, but macros do not. Using const can help prevent subtle data-dependent errors that are often difficult to debug.
I think the main advantage is that you can change the constant without having to recompile everything that uses it.
Since a macro change will effectively modify the contents of the file that use the macro, recompilation is necessary.
In C the const qualifier does not define a constant but instead a read-only object:
#define A 42 // A is a constant
const int a = 42; // a is not constant
A const object cannot be used where a real constant is required, for example:
static int bla1 = A; // OK, A is a constant
static int bla2 = a; // compile error, a is not a constant
Note that this is different in C++ where the const really qualifies an object as a constant.
The only problems you list with const sum up as "I've got the most incompetent compiler I can possibly imagine". The problems with #define, however, are universal- for example, no scoping.
There's no reason to use #define instead of a const int in C++. Any decent C++ compiler will substitute the constant value from a const int in the same way it does for a #define where it is possible to do so. Both take approximately the same amount of flash when used the same way.
Using a const does allow you to take the address of the value (where a macro does not). At that point, the behavior obviously diverges from the behavior of a Macro. The const now needs a space in the program in both flash and in RAM to live so that it can have an address. But this is really what you want.
The overhead here is typically going to be an extra 8 bytes, which is tiny compared to the size of most programs. Before you get to this level of optimization, make sure you have exhausted all other options like compiler flags. Using the compiler to carefully optimize for size and not using things like templates in C++ will save you a lot more than 8 bytes.
I was asked this question in a technical interview:
What is the difference between a const and a macro in C++?
My answer was that a macro is a preprocessor directive and it could be difficult to debug the application if you use a macro since it is replaced with the constant expression before compilation, whereas a const can have a type identifier and is easy to debug.
Could anyone point out any other difference and which should be preferred?
EDIT:
From the IBM documentation for C++:
The following are some differences between #define and the const type qualifier:
The #define directive can be used to create a name for a numerical, character, or string constant, whereas a const object of any type can be declared.
A const object is subject to the scoping rules for variables, whereas a constant created using #define is not. Unlike a const object, the value of a macro does not appear in the intermediate source code used by the compiler because they are expanded inline. The inline expansion makes the macro value unavailable to the debugger.
A macro can be used in a constant expression, such as an array bound, whereas a const object cannot. (I think we surely need to use macro to define array_size.
The compiler does not type-check a macro, including macro arguments.
Macros and constants are not remotely the same thing, each is sometimes appropriate for the circumstances, and your answer only scratches at the surface of the difference. Also, C++ has two different kinds of constants.
A constant defined with the const qualifier is best thought of as an unmodifiable variable. It has all the properties of a variable: it has a type, it has a size, it has linkage, you can take its address. (The compiler might optimize away some of these properties if it can get away with it: for instance, constants whose address is never used may not get emitted into the executable image. But this is only by the grace of the as-if rule.) The only thing you can't do to a const datum is change its value. A constant defined with enum is a little different. It has a type and a size, but it doesn't have linkage, you can't take its address, and its type is unique. Both of these are processed during translation phase 7, so they can't be anything but an lvalue or rvalue. (I'm sorry about the jargon in the preceding sentence, but I would have to write several paragraphs otherwise.)
A macro has far fewer constraints: it can expand to any sequence of tokens, as long as the overall program remains a well-formed program. It doesn't have any of the properties of a variable. Applying sizeof or & to a macro may or may not do something useful, depending on what the macro expands to. Macros are sometimes defined to expand to numeric literals, and such macros are sometimes thought of as constants, but they're not: "the compiler proper" (that is, translation phase 7) sees them as numeric literals.
It is generally considered good practice, nowadays, not to use a macro when a constant will do. Macros don't obey the same scoping rules as all other identifiers, which can be confusing, and if you use a constant you give more information to translation phase 7 and thus also to the debugger. However, macros permit you to do things that cannot be done any other way, and if you need to do one of those things, you should not hesitate to use them. (Macros that are pulling their weight, in this sense, generally do not just expand to numeric literals, though I am not going to say never.)
EDIT: Here's an example of a macro doing something interesting. It is in no way, shape or form a constant. There may well be a way to get the same effect without a macro (if you know one that doesn't involve stringstreams, I'd be curious to hear about it!) but I think it makes a good illustration of both the power and the danger of macros (for the latter, consider what it would do if it was used outside of one very specific context...)
static double elapsed()
{ ... }
#define ELAPSED '[' << std::fixed << std::setprecision(2) << elapsed() << "] "
// usage:
for (vector<string>::iterator f = files.begin(); f != files.end(); f++) {
cout << ELAPSED << "reading file: " << *f << '\n';
process_file(*f);
}
One should prefer const int sum = 1; over #define sum 1 for a number of reasons:
Scope Based Mechanism:
#defines don't respect scopes so there is no way to create a class scoped namespace. While const variables can be scoped in classes.
Avoiding Weird magical numbers during compilation errors:
If you are using #define those are replaced by the pre-processor at time of precompilation So if you receive an error during compilation, it will be confusing because the error message wont refer the macro name but the value and it will appear a sudden value, and one would waste lot of time tracking it down in code.
Ease of Debugging:
Also for same reasons, while debugging #define would provide no help really.
To avoid both above situations const will be a better choice.
( Originally posted for static const vs #define - reproducing here as this question seems to have more "momentum"... let me know if that's inappropriate... )
Pros and cons to everything, depending on usage:
consts
properly scoped / identifier clash issues handled nicely
strong, single, user-specified type
you might try to "type" a #define ala #define S std::string("abc"), but the constant avoids repeated construction of distinct temporaries at each point of use
One Definition Rule complications
can take address, create const references to them etc.
defines
"global" scope / more prone to conflicting usages, which can produce hard-to-resolve compilation issues and unexpected run-time results rather than sane error messages; mitigating this requires:
long, obscure and/or centrally coordinated identifiers, and access to them can't benefit from implicitly matching used/current/Koenig-looked-up namespace, namespace aliases etc.
use of all uppercase characters is generally required and reserved for preprocessor defines (an important guideline for enterprise scale preprocessor usage to remain manageable, and which 3rd party libraries can be expected to follow), observation of which implies migration of existing consts or enums to defines involves a change in capitalisation (and hence affects client code). (Personally, I capitalise the first letter of enums but not consts, so I'd be hit here anyway - maybe time to rethink that.)
more compile-time operations possible: string literal concatenation, stringification (taking size thereof)
downside is that given #define X "x" and some client usage ala "pre" X "post", you're in trouble if you want or need to make X a runtime-changeable variable rather than a constant, whereas that transition is easier from a const char* or const std::string given they already force the user to incorporate concatenation operations.
can't use sizeof directly on a defined numeric constant
untyped (GCC doesn't warn if compared to unsigned)
some compiler/linker/debugger chains may not present the identifier, so you'll be reduced to looking at "magic numbers" (strings, whatever...)
can't take the address
the substituted value need not be legal (or discrete) in the context where the #define is created, as it's evaluated at each point of use, so you can reference not-yet-declared objects, depend on "implementation" that needn't be pre-included, create "constants" such as { 1, 2 } that can be used to initialise arrays, or #define MICROSECONDS *1E-6 etc. (definitely not recommending this!)
some special things like __FILE__ and __LINE__ can be incorporated into the macro substitution
enums
only possible for integer values
properly scoped / identifier clash issues handled nicely
strongly typed, but to a big-enough signed-or-unsigned int size over which you have no control (in C++03)
can't take the address
stronger usage restraints (e.g. incrementing - template <typename T> void f(T t) { cout << ++t; } won't compile)
each constant's type taken from the enclosing enum, so template <typename T> void f(T) get a distinct instantiation when passed the same numeric value from different enums, all of which are distinct from any actual f(int) instantiation.
even with typeof, can't expect numeric_limits to provide useful insight
the enum's typename may appear in various places in RTTI, compiler messages etc. - possibly useful, possibly obfuscation
As a general rule, I use consts and consider them the most professional option for general usage (though the others have a simplicity appealing to this old lazy programmer).
Another difference is that a const variable has a memory and can be referenced by a pointer. Macro is just the autocomplete that will happen before compilation, hence the name is lost during compiling.
Also macro can be just more than a constant. It can be am expression or anything that is syntactically correct, even a whole definition of a function.
Macros are used to depict programming choices e.g. stack size; while cosnt is used to depict the real world constants like value of Pi or e.
define can be redefine , but const will be cause compiler error:
sample:
source : main.cpp
#define int_constance 4
#define int_constance 8 // ok, compiler will warning ( redefine macro)
const int a = 2;
const int a = 4; // redefine -> error
int main(int argc, char** argv)
{
std::cout << int_constance ; // if remove second #define line, output will be 8
return 0;
}
Macros don't respect scope, and a macro's name may not be available to a symbolic debugger. Dan Saks has a fairly complete article on the relative merits of macros (none), constant objects, and enumeration constants. Like Stephen Dewhurst, Saks prefers enumeration constants for integer values since they take up no storage (more precisely, enumeration constants have neither storage duration nor linkage).
A macro always have a type, for instance, #define FIVE 5 is of type int.
An advantage for the const variable over the macro could be the memory usage : With a macro the value may have to be duplicated everywhere it is used will a const variable will not be duplicated in memory. (but I am not sure of this difference)