static const vs define

Question

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

User · Answer

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.

User · Answer

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  lt typename T gt  void f T t    cout  lt  lt    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  lt typename T gt  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

User · Answer

Please see here   static const vs define  usually a const declaration  notice it doesn t need to be static  is the way to go

User · Answer

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

User · Answer

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

User · Answer

Always prefer to use the language features over some additional tools like preprocessor   ES 31  Don t use macros for constants or  quot functions quot  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

User · Answer

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

User · Answer

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

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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

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define can lead to unexpected results    include  lt iostream gt    define x 500  define y x   5  int z   y   2   int main         std  cout  lt  lt   y is    lt  lt  y      std  cout  lt  lt    nz is    lt  lt  z      Outputs an incorrect result   y is 505 z is 510   However  if you replace this with constants    include  lt iostream gt   const int x   500  const int y   x   5   int z   y   2   int main         std  cout  lt  lt   y is    lt  lt  y      std  cout  lt  lt    nz is    lt  lt  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

User · Answer

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

[c++] static const vs #define

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