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).
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.
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)
What's the difference between using a define statement and an enum statement in C/C++ (and is there any difference when using them with either C or C++)?
For example, when should one use
enum {BUFFER = 1234};
over
#define BUFFER 1234
enum defines a syntactical element.
#define is a pre-preprocessor directive, executed before the compiler sees the code, and therefore is not a language element of C itself.
Generally enums are preferred as they are type-safe and more easily discoverable. Defines are harder to locate and can have complex behavior, for example one piece of code can redefine a #define made by another. This can be hard to track down.
#define statements are handled by the pre-processor before the compiler gets to see the code so it's basically a text substitution (it's actually a little more intelligent with the use of parameters and such).
Enumerations are part of the C language itself and have the following advantages.
1/ They may have type and the compiler can type-check them.
2/ Since they are available to the compiler, symbol information on them can be passed through to the debugger, making debugging easier.
Enums are generally prefered over #define wherever it makes sense to use an enum:
Debuggers can show you the symbolic name of an enums value ("openType: OpenExisting", rather than "openType: 2"
You get a bit more protection from name clashes, but this isn't as bad as it was (most compilers warn about re#defineition.
The biggest difference is that you can use enums as types:
// Yeah, dumb example
enum OpenType {
OpenExisting,
OpenOrCreate,
Truncate
};
void OpenFile(const char* filename, OpenType openType, int bufferSize);
This gives you type-checking of parameters (you can't mix up openType and bufferSize as easily), and makes it easy to find what values are valid, making your interfaces much easier to use. Some IDEs can even give you intellisense code completion!
Define is a preprocessor command, it's just like doing "replace all" in your editor, it can replace a string with another and then compile the result.
Enum is a special case of type, for example, if you write:
enum ERROR_TYPES
{
REGULAR_ERR =1,
OK =0
}
there exists a new type called ERROR_TYPES.
It is true that REGULAR_ERR yields to 1 but casting from this type to int should produce a casting warning (if you configure your compiler to high verbosity).
Summary:
they are both alike, but when using enum you profit the type checking and by using defines you simply replace code strings.
It's always better to use an enum if possible. Using an enum gives the compiler more information about your source code, a preprocessor define is never seen by the compiler and thus carries less information.
For implementing e.g. a bunch of modes, using an enum makes it possible for the compiler to catch missing case-statements in a switch, for instance.
enum can group multiple elements in one category:
enum fruits{ apple=1234, orange=12345};
while #define can only create unrelated constants:
#define apple 1234
#define orange 12345
#define is a preprocessor command, enum is in the C or C++ language.
It is always better to use enums over #define for this kind of cases. One thing is type safety. Another one is that when you have a sequence of values you only have to give the beginning of the sequence in the enum, the other values get consecutive values.
enum {
ONE = 1,
TWO,
THREE,
FOUR
};
instead of
#define ONE 1
#define TWO 2
#define THREE 3
#define FOUR 4
As a side-note, there is still some cases where you may have to use #define (typically for some kind of macros, if you need to be able to construct an identifier that contains the constant), but that's kind of macro black magic, and very very rare to be the way to go. If you go to these extremities you probably should use a C++ template (but if you're stuck with C...).
If you only want this single constant (say for buffersize) then I would not use an enum, but a define. I would use enums for stuff like return values (that mean different error conditions) and wherever we need to distinguish different "types" or "cases". In that case we can use an enum to create a new type we can use in function prototypes etc., and then the compiler can sanity check that code better.
Besides all the thing already written, one said but not shown and is instead interesting. E.g.
enum action { DO_JUMP, DO_TURNL, DO_TURNR, DO_STOP };
//...
void do_action( enum action anAction, info_t x );
Considering action as a type makes thing clearer. Using define, you would have written
void do_action(int anAction, info_t x);
For integral constant values I've come to prefer enum over #define. There seem to be no disadvantages to using enum (discounting the miniscule disadvantage of a bit more typing), but you have the advantage that enum can be scoped, while #define identifiers have global scope that tromps everything.
Using #define isn't usually a problem, but since there are no drawbacks to enum, I go with that.
In C++ I also generally prefer enum to const int even though in C++ a const int can be used in place of a literal integer value (unlike in C) because enum is portable to C (which I still work in a lot) .
If you have a group of constants (like "Days of the Week") enums would be preferable, because it shows that they are grouped; and, as Jason said, they are type-safe. If it's a global constant (like version number), that's more what you'd use a #define for; although this is the subject of a lot of debate.
In addition to the good points listed above, you can limit the scope of enums to a class, struct or namespace. Personally, I like to have the minimum number of relevent symbols in scope at any one time which is another reason for using enums rather than #defines.
Another advantage of an enum over a list of defines is that compilers (gcc at least) can generate a warning when not all values are checked in a switch statement. For example:
enum {
STATE_ONE,
STATE_TWO,
STATE_THREE
};
...
switch (state) {
case STATE_ONE:
handle_state_one();
break;
case STATE_TWO:
handle_state_two();
break;
};
In the previous code, the compiler is able to generate a warning that not all values of the enum are handled in the switch. If the states were done as #define's, this would not be the case.
enums are more used for enumerating some kind of set, like days in a week. If you need just one constant number, const int (or double etc.) would be definetly better than enum. I personally do not like #define (at least not for the definition of some constants) because it does not give me type safety, but you can of course use it if it suits you better.
Creating an enum creates not only literals but also the type that groups these literals: This adds semantic to your code that the compiler is able to check.
Moreover, when using a debugger, you have access to the values of enum literals. This is not always the case with #define.
While several answers above recommend to use enum for various reasons, I'd like to point out that using defines has an actual advantage when developing interfaces. You can introduce new options and you can let software use them conditionally.
For example:
#define OPT_X1 1 /* introduced in version 1 */
#define OPT_X2 2 /* introduced in version 2 */
Then software which can be compiled with either version it can do
#ifdef OPT_X2
int flags = OPT_X2;
#else
int flags = 0;
#endif
While on an enumeration this isn't possible without a run-time feature detection mechanism.
Enum:
1. Generally used for multiple values
2. In enum there are two thing one is name and another is value of name name must be distinguished but value can be same.If we not define value then first value of enum name is 0 second value is 1,and so on, unless explicitly value are specified.
3. They may have type and compiler can type check them
4. Make debugging easy
5. We can limit scope of it up to a class.
Define:
1. When we have to define only one value
2. It generally replace one string to another string.
3. It scope is global we cannot limit its scope
Overall we have to use enum
There is little difference. The C Standard says that enumerations have integral type and that enumeration constants are of type int, so both may be freely intermixed with other integral types, without errors. (If, on the other hand, such intermixing were disallowed without explicit casts, judicious use of enumerations could catch certain programming errors.)
Some advantages of enumerations are that the numeric values are automatically assigned, that a debugger may be able to display the symbolic values when enumeration variables are examined, and that they obey block scope. (A compiler may also generate nonfatal warnings when enumerations are indiscriminately mixed, since doing so can still be considered bad style even though it is not strictly illegal.) A disadvantage is that the programmer has little control over those nonfatal warnings; some programmers also resent not having control over the sizes of enumeration variables.