I have a C project where all code is organized in *.c/*.h file pairs, and I need to define a constant value in one file, which will be however also be used in other files. How should I declare and define this value?
Should it be as static const ... in the *.h file? As extern const ... in the *.h file and defined in the *.c file? In what way does it matter if the value is not a primitive datatype (int, double, etc), but a char * or a struct? (Though in my case it is a double.)
Defining stuff inside *.h files doesn't seem like a good idea generally; one should declare things in the *.h file, but define them in the *.c file. However, the extern const ... approach seems inefficient, as the compiler wouldn't be able to inline the value, it instead having to be accessed via its address all the time.
I guess the essence of this question is: Should one define static const ... values in *.h files in C, in order to use them in more that one place?
The rule I follow is to only declare things in H files and define them in C files. You can declare and define in a single C file, assuming it will only be used in that file.
By declaration, I mean notify the compiler of its existence but don't allocate space for it. This includes #define, typedef, extern int x, and so on.
Definitions assign values to declarations and allocate space for them, such as int x and const int x. This includes function definitions; including these in header files frequently lead to wasted code space.
I've seen too many junior programmers get confused when they put const int x = 7; in a header file and then wonder why they get a link error for x being defined more than once. I think at a bare minimum, you would need static const int x so as to avoid this problem.
I wouldn't be too worried about the speed of the code. The main issue with computers (in terms of speed and cost) long ago shifted from execution speed to ease of development.
If you need constants (real, compile time constants) you can do that three ways, putting them into header files (there is nothing bad with that):
enum {
FOO_SIZE = 1234,
BAR_SIZE = 5678
};
#define FOO_SIZE 1234
#define BAR_SIZE 5678
static const int FOO_SIZE = 1234;
static const int BAR_SIZE = 5678;
In C++, i tend to use the enum way, since it can be scoped into a namespace. For C, i use the macro. This basicially comes down to a matter of taste though. If you need floating point constants, you can't use the enumeration anymore. In C++ i use the last way, the static const double, in that case (note in C++ static would be redundant then; they would become static automatically since they are const). In C, i would keep using the macros.
It's a myth that using the third method will slow down your program in any way. I just prefer the enumeration since the values you get are rvalues - you can't get their address, which i regard as an added safety. In addition, there is much less boiler-plate code written. The eye is concentrated on the constants.
Do you really have a need to worry about the advantage of inline? Unless you're writing embedded code, stick to readability. If it's really a magic number of something, I'd use a define; I think const is better for things like const version strings and modifying function call arguments. That said, the define in .c, declare in .h rule is definitely a fairly universally accepted convention, and I wouldn't break it just because you might save a memory lookup.
As a general rule, you do not define things as static in a header. If you do define static variables in a header, each file that uses the header gets its own private copy of whatever is declared static, which is the antithesis of DRY principle: don't repeat yourself.
So, you should use an alternative. For integer types, using enum (defined in a header) is very powerful; it works well with debuggers too (though the better debuggers may be able to help with #define macro values too). For non-integer types, an extern declaration (optionally qualified with const) in the header and a single definition in one C file is usually the best way to go.
I'd like to see more context for your question. The type of the value is critical, but you've left it out. The meaning of the const keyword in C is quite subtle; for example
const char *p;
does not mean that pointer p is a constant; you can write p all you like. What you cannot write is the memory that p points to, and this stays true even as p's value changes. This is about the only case I really understand; in general, the meaning of the subtle placement of const eludes me. But this special case is extremely useful for function parameters because it extracts a promise from the function that the memory the argument points to will not be mutated.
There is one other special case everyone should know: integers. Almost always, constant, named integers should be defined in a .h file as enumeration literals. enum types not only allow you to group related constants together in a natural way, but also allow you the names of those constants to be seen in the debugger, which is a huge advantage.
I've written tens of thousands of lines of C; probably hundreds if I try to track it down. (wc ~/src/c/*.c says 85 thousand, but some of that is generated, and of course there's a lot of C code lurking elsewhere). Aside from the two cases about, I've never found much use for const. I would be pleased to learn a new, useful example.
I can give you an indirect answer. In C++ (as opposed to C) const implies static. Thatis to say in C++ static const is the same thing as const. So that tells you how that C++ standards body feels about the issue i.e. all consts should be static.
for autoconf environment:
You can always define constants in the configure file as well. AC_DEFINE() i guess is the macro to define across the entire build.
To answer the essence of your question:
You generally do NOT want to define a static variable in a header file.
This would cause you to have duplicated variables in each translation units (C files) that include the header.
variables in a header should really be declared extern since that is the implied visibility.
See this question for a good explanation.
Actually, the situation might not be so dire, as the compiler would probably convert a const type to a literal value. But you might not want to rely on that behavior, especially if optimizations are turned off.
In C++, you should always use
const int SOME_CONST = 17;
for constants and never
#define SOME_CONST 17
Defines will almost always come back and bite you later. Consts are in the language, and are strongly typed, so you won't get weird errors because of some hidden interaction. I would put the const in the appropriate header file. As long as it's #pragma once (or #ifndef x / #define x / #endif), you won't ever get any compile errors.
In vanilla C, you might have compatibility problems where you must use #defines.
Related
I'm trying to use rlutil.h but everytime these function are used in more than one header I have compiler error about multiple definition of 20-30 variables. rlutil is a simple header to color terminal in linux and windows in C and C++.
The variables are something like that
const RLUTIL_STRING_T ANSI_CONSOLE_TITLE_POST = "\007";
and the typedef something like that
typedef const char* RLUTIL_STRING_T;
I tried to add my own C guard but it didn't worked.
I tried to layering the .h with my own .h/.c to make new function using the rlutil.h function but the problem is still here.
I tried to make the variables extern but it's worst
I'm building it with gcc on ubuntu.
I'm gonna try this at home with MVSC2017 but I think the behavior will be the same.
Any idea ?
I can provide more information.
Sorry for my english i'm not a native speaker
Thank's a lot
The problem is that the header is only set up so that it works with C++, where the const values defined in the header rlutil.h are private to each translation unit (TU) — think source file plus headers — that includes the header. By contrast, in C, they are normal global variables defined in each TU that includes rlutil.h, leading to the multiple definitions problem.
There isn't a trivial fix — unless switching from C to C++ is deemed trivial. The header attempts to be language-neutral between C and C++, but it fails on this count. Once again, proof that C and C++ are different languages.
In C, you would need to have code like:
extern const RLUTIL_STRING_T ANSI_CONSOLE_TITLE_POST;
in the header and then one source file would define the values:
const RLUTIL_STRING_T ANSI_CONSOLE_TITLE_POST = "\007"; // James Bond!
Alternatively, you could consider using static in the header:
static const RLUTIL_STRING_T ANSI_CONSOLE_TITLE_POST = "\007";
Each C file that includes this header would have its own collection of defined variables. In C, you'd be subject to compiler warnings about unused variables, which is not desirable. In C++, you might get warnings about using static instead of an anonymous namespace. It isn't clear that this is a good solution, therefore.
If you're brave, you could read the tail end of my answer to How do I use extern to share variables between files, but the header is probably not in your control and you really need to report the trouble to the maintainers of the code. (If you are the maintainer, then think about whether a scheme such as that outlined in the answer to the other question will help.)
This is a case of "static const” vs “#define” in C" for embedded systems.
On large/mid projects with "passed-down" code and modules, what is the best practice on writing constant parameters for your include files, modules, etc?
In a code "passed-down" where you don't know if the names you're choosing are defined in some other included file or might be called with extern or as macros in some other file that might include your file.
Having these 3 options:
static const int char_height = 12;
#define CHAR_HEIGHT 12
enum { char_height = 12 };
which one would be better (on an embedded system with unknown memory constraints)?
The original code uses mainly #define's for this, but these kind of constants are haphazardly implemented in several ways (and at different locations even in the same files) since it seems several people developed this demo software for a certain device.
Specifically, this is a demo code, showing off every hardware and SDK feature of a certain device.
Most of the data I'm thinking about is the kind used to configure the environment: screen dimensions, charset characteristics, something to improve the readability of the code. Not on the automatic configuration a compiler and pre-processor could do. But since there's a lot of code in there and I'm afraid of global name conflicts, I'm reluctant to use #define's
Currently, I'm considering that it would be better to rewrite the project from scratch and re-implement most of the already written functions to get their constants from just one c file or reorganize the constants' implementation to just one style.
But:
This is a one person project (so it would take a lot of time to re-implement everything)
The already implemented code works and it has been revised several times. (If it's not broken...)
Always consider readability and memory constraints. Also, macros are simply copy/paste operations that occur before compilation. With that being said I like to do the following:
I define all variables that are constant as being static const if they are to be used in one c file (e.g. not globally accessible across multiple files). Anything defined as const shall be placed in ROM when at file scope. Obviously you cannot change these variables after they're initialized.
I define all constant values using #define.
I use enumerations where it adds to readability. Any place where you have a fixed range of values I prefer enumerations to explicitly state the intent.
Try to approach the project with an object oriented perspective (even though c isn't OO). Hide private functions (don't create a prototype in the header), do not use globals if you can avoid it, mark variables that should only reside in one c module (file) as static, etc.
They are 3 different things that should be used in 3 different situations.
#define should be used for constants that need to be evaluated at compile time. One typical example is the size of a statically allocated array, i.e.
#define N 10
int x[N];
It is also fine to use #define all constants where it doesn't matter how or where the constant is allocated. People who claim that it is bad practice to do so only voice their own, personal, subjective opinions.
But of course, for such cases you can also use const variables. There is no important difference between #define and const, except for the following cases:
const should be used where it matters at what memory address a constant is allocated. It should also be used for variables that the programmer will likely change often. Because if you used const, you an easily move the variable to a memory segment in EEPROM or data flash (but if you do so, you need to declare it as volatile).
Another slight advantage of const is that you get stronger type safety than a #define. For the #define to get equal type safety, you have to add explicit type casts in the macro, which might get a bit harder to read.
And then of course, since consts (and enums) are variables, you can reduce their scope with the static keyword. This is good practice since such variables do not clutter down the global namespace. Although the true source of name conflicts in the global namespaces are in 99% of all cases caused by poor naming policies, or no naming policies at all. If you follow no coding standard, then that is the true source of the problem.
So generally it is fine to make constants global when needed, it is rather harmless practice as long as you have a sane naming policy (preferably all items belonging to one code module should share the same naming prefix). This shouldn't be confused with the practice of making regular variables global, which is always a very bad idea.
Enums should only be used when you have several constant values that are related to each other and you want to create a special type, such as:
typedef enum
{
OK,
ERROR_SOMETHING,
ERROR_SOMETHING_ELSE
} error_t;
One advantage of the enum is that you can use a classic trick to get the number of enumerated items as another compile-time constant "free of charge":
typedef enum
{
OK,
ERROR_SOMETHING,
ERROR_SOMETHING_ELSE,
ERRORS_N // the number of constants in this enum
} error_t;
But there are various pitfalls with enums, so they should always be used with caution.
The major disadvantage of enum is that it isn't type safe, nor is it "type sane". First of all, enumeration constants (like OK in the above example) are always of the type int, which is signed.
The enumerated type itself (error_t in my example) can however be of any type compatible with char or int, signed or unsigned. Take a guess, it is implementation-defined and non-portable. Therefore you should avoid enums, particularly as part of various data byte mappings or as part of arithmetic operations.
I agree with bblincoe...+1
I wonder if you understand what the differences are in that syntax and how it can/might affect implementation. Some folks may not care about implementation but if you are moving into embedded perhaps you should.
When bblincoe mentions ROM instead of RAM.
static const int char_height = 12;
That should, ideally, consume .text real estate and pre-init that real estate with the value you specified. Being const you wont change it but it does have a placeholder? now why would you need a placeholder for a constant? think about that, certainly you could hack the binary down the road for some reason to turn something on or off or change a board specific tuning parameter...
Without a volatile though that doesnt mean that compiler has to always use that .text location, it can optimize and put that value in as instructions directly or even worse optimize math operations and remove some math.
The define and enum do not consume storage, they are constants that the compiler chooses how to implement, ultimately those bits if they are not optimized away, land somewhere in .text sometimes everywhere in .text, depends on the instruction set how its immediates work the specific constant, etc.
So define vs enum is basically do you want to pick all the values or do you want the compiler to pick some values for you, define if you want to control it enum if you want the compiler to choose the values.
So it really isnt a best practice thing at all it is a case of determining what your program needs to do and choosing the appropriate programming solution for that situation.
Depending on the compiler and the target processor, choosing volatile static const int vs not doing that can affect the rom consumption. But it is a very specific optimization, and not a general answer (and has nothing to do with embedded but with compiling in general).
Dan Saks explains why he prefers the enumeration constant in these articles, Symbolic Constants and Enumeration Constants vs Constant Objects. In summary, avoid macros because they don't observe the usual scope rules and the symbolic names are typically not preserved for symbolic debuggers. And prefer enumeration constants because they are not susceptible to a performance penalty that may affect constant objects. There is a lot more details in the linked articles.
Another thing to considerer is performance. A #define constant can usually be accessed faster than a const variable (for integers) since the const will need to be fetched from ROM (or RAM) and the #define value will usually be an immediate instruction argument so it is fetched along with the instruction (no extra cycles).
As for naming conflicts, I like to use prefixes like MOD_OPT_ where MOD is the module name OPT means that the define is a compile-time option, etc. Also only include the #defines in your header files if they're part of the public API, otherwise use an .inc file if they're needed in multiple source files or define them in the source file itself if they're only specific to that file.
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"static const" vs "#define" vs "enum"
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In many programs a #define serves the same purpose as a constant. For example.
#define FIELD_WIDTH 10
const int fieldWidth = 10;
I commonly see the first form preferred over the other, relying on the pre-processor to handle what is basically an application decision. Is there a reason for this tradition?
There is a very solid reason for this: const in C does not mean something is constant. It just means a variable is read-only.
In places where the compiler requires a true constant (such as for array sizes for non-VLA arrays), using a const variable, such as fieldWidth is just not possible.
They're different.
const is just a qualifier, which says that a variable cannot be changed at runtime. But all other features of the variable persist: it has allocated storage, and this storage may be addressed. So code does not just treat it as a literal, but refers to the variable by accessing the specified memory location (except if it is static const, then it can be optimized away), and loading its value at runtime. And as a const variable has allocated storage, if you add it to a header and include it in several C sources, you'll get a "multiple symbol definition" linkage error unless you mark it as extern. And in this case the compiler can't optimize code against its actual value (unless global optimization is on).
#define simply substitutes a name with its value. Furthermore, a #define'd constant may be used in the preprocessor: you can use it with #ifdef to do conditional compilation based on its value, or use the stringizing operator # to get a string with its value. And as the compiler knows its value at compile time it may optimize code based on that value.
For example:
#define SCALE 1
...
scaled_x = x * SCALE;
When SCALE is defined as 1 the compiler can eliminate the multiplication as it knows that x * 1 == x, but if SCALE is an (extern) const, it will need to generate code to fetch the value and perform the multiplication because the value will not be known until the linking stage. (extern is needed to use the constant from several source files.)
A closer equivalent to using #define is using enumerations:
enum dummy_enum {
constant_value = 10010
};
But this is restricted to integer values and doesn't have advantages of #define, so it is not widely used.
const is useful when you need to import a constant value from some library where it was compiled in. Or if it is used with pointers. Or if it is an array of constant values accessed through a variable index value. Otherwise, const has no advantages over #define.
The reason is that most of the time, you want a constant, not a const-qualified variable. The two are not remotely the same in the C language. For example, variables are not valid as part of initializers for static-storage-duration objects, as non-vla array dimensions (for example the size of an array in a structure, or any array pre-C99).
Expanding on R's answer a little bit: fieldWidth is not a constant expression; it's a const-qualified variable. Its value is not established until run-time, so it cannot be used where a compile-time constant expression is required (such as in an array declaration, or a case label in a switch statement, etc.).
Compare with the macro FIELD_WIDTH, which after preprocessing expands to the constant expression 10; this value is known at compile time, so it can be used for array dimensions, case labels, etc.
To add to R.'s and Bart's answer: there is only one way to define symbolic compile time constants in C: enumeration type constants. The standard imposes that these are of type int. I personally would write your example as
enum { fieldWidth = 10 };
But I guess that taste differs much among C programmers about that.
Although a const int will not always be appropriate, an enum will usually work as a substitute for the #define if you are defining something to be an integral value. This is actually my preference in such a case.
enum { FIELD_WIDTH = 16384 };
char buf[FIELD_WIDTH];
In C++ this is a huge advantage as you can scope your enum in a class or namespace, whereas you cannot scope a #define.
In C you don't have namespaces and cannot scope an enum inside a struct, and am not even sure you get the type-safety, so I cannot actually see any major advantage, although maybe some C programmer there will point it out to me.
According to K&R (2nd edition, page 211) the "const and volatile properties are new with the ANSI standard". This may imply that really old ANSI code did not have these keywords at all and it really is just a matter of tradition.
Moreover, it says that a compiler should detect attempts to change const variables but other than that it may ignore these qualifiers. I think it means that some compilers may not optimize code containing const variable to be represented as intermediate value in machine code (like #define does) and this might cost in additional time for accessing far memory and affect performance.
Some C compilers will store all const variables in the binary, which if preparing a large list of coefficients can use up a tremendous amount of space in the embedded world.
Conversely: using const allows flashing over an existing program to alter specific parameters.
The best way to define numeric constants in C is using enum. Read the corresponding chapter of K&R's The C Programming Language, page 39.
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.
In C, shall I prefer constants over defines? I've reading a lot of code lately, and all of the examples make heavy use of defines.
No, in general you should not use const-qualified objects in C to create names constants. In order to create a named constant in C you should use either macros (#define) or enums. In fact, C language has no constants, in the sense that you seem to imply. (C is significantly different from C++ in this regard)
In C language the notions of constant and constant expression are defined very differently from C++. In C constant means a literal value, like 123. Here are some examples of constants in C
123
34.58
'x'
Constants in C can be used to build constant expressions. However, since const-qualified objects of any type are not a constants in C, they cannot be used in constant expressions, and, consequently, you cannot use const-qualified objects where constant expressions are required.
For example, the following is not a constant
const int C = 123; /* C is not a constant!!! */
and since the above C is not a constant, it cannot be used to declare an array type in file scope
typedef int TArray[C]; /* ERROR: constant expression required */
It cannot be used as a case label
switch (i) {
case C: ; /* ERROR: constant expression required */
}
It cannot be used as bit-field width
struct S {
int f : C; /* ERROR: constant expression required */
};
It cannot be used as an initializer for an object with static storage duration
static int i = C; /* ERROR: constant expression required */
It cannot be used as a enum initializer
enum {
E = C /* ERROR: constant expression required */
};
i.e it cannot be used anywhere where a constant is required.
This might seem counter-intuitive, but this is how C the language is defined.
This is why you see these numerous #define-s in the code you are working with. Again, in C language const-qualified object have very limited use. They are basically completely useless as "constants", which is why in C language you are basically forced to use #define or enums to declare true constants.
Of course, in situations when a const-qualified object works for you, i.e. it does what you want it to do, it is indeed superior to macros in many ways, since it is scoped and typed. You should probably prefer such objects where applicable, however in general case you'll have to take into account the above limitations.
Constants should be preferred over defines. There are several advantages:
Type safety. While C is a weakly typed languaged, using a define loses all of the type safety, which will allow the compiler to pick up problems for you.
Ease of debugging. You can change the value of constants through the debugger, while defines are automatically changed in the code by the pre-processor to the actual value, meaning that if you want to change the value for test/debugging purposes, you need to re-compile.
Maybe I have been using them wrong but, at least in gcc, you can't use constants in case statements.
const int A=12;
switch (argc) {
case A:
break;
}
Though this question is specific to C, I guess it is good to know this:
#include<stdio.h>
int main() {
const int CON = 123;
int* A = &CON;
(*A)++;
printf("%d\n", CON); // 124 in C
}
works in C, but not in C++
One of the reasons to use #define is to avoid such things to mess up your code, specially it is a mix of C and C++.
A lot of people here are giving you "C++ style" advice. Some even say the C++ arguments apply to C. That may be a fair point. (Whether it is or not feels kind of subjective.) The people who say const sometimes means something different in the two languages are also correct.
But these are mostly minor points and personally, I think in truth there is relatively minor consequence to going either way. It's a matter of style, and I think different groups of people will give you different answers.
In terms of common usage, historical usage, and most common style, in C, it's a lot more typical to see #define. Using C++isms in C code can come off as strange to a certain narrow segment of C coders. (Including me, so that's where my biases lie.)
But I'm surprised no one has suggested a middle ground solution, that "feels right" in both languages: if it fits into a group of integer constants, use an enum.
define can be used for many purposes(very loose) and should be avoided if you can substitute that with const, which define a variable and you can do a lot more with it.
In cases like below, define has to be used
directive switch
substitution to your source line
code macros
An example where you have to use define over const is when you have version number say 3 and you want version 4 to include some methods that is not available in version 3
#define VERSION 4
...
#if VERSION==4
................
#endif
Defines have been part of the language longer than constants, so a lot of older code will use them because defines where the only way to get the job done when the code was written. For more recent code it may be simply a matter of programmer habit.
Constants have a type as well as a value, so they would be preferred when it makes sense for your value to have a type, but not when it is typeless (or polymorphic).
If it's something that isn't determined programmatically, I use #define. For example, if I want all of my UI objects to have the same space between them, I might use #define kGUISpace 20.
Apart from the excellent reasons given by AndreyT for using DEFINES rather than constants in "C" code there is another more pragmatic reason for using DEFINES.
DEFINES are easy define and use from (.h) header files, which is where any experienced C coder would expect to find constants defined. Defining consts in header files is not quite so easy -- its more code to avoid duplicate definitions etc.
Also the "typesafe" arguments are moot most compilers will pick up glaring errors suchh as assing a string to and int, or, will "do the right thing" on a slight mismatch such as assigning an integer to a float.
Macros (defines) can be used by the pre-processor and at compile time, constants cannot.
You can do compile-time checks to make sure a macro is within a valid range (and #error or #fatal if it isn't). You can use default values for a macro if it hasn't already been defined. You can use a macro in the size of an array.
A compiler can optimize with macros better than it can with constants:
const int SIZE_A = 15;
#define SIZE_B 15
for (i = 0; i < SIZE_A + 1; ++i); // if not optimized may load A and add 1 on each pass
for (i = 0; i < SIZE_B + 1; ++i); // compiler will replace "SIZE_B + 1" with 16
Most of my work is with embedded processors that don't have amazing optimizing compilers. Maybe gcc will treat SIZE_A like a macro at some optimization level.