How do I check if a variable is of a certain type compare two types in C

Question

In C  not C   C   how do I check if a variable is of a certain type   For example  something like this   double doubleVar  if  typeof doubleVar     double         printf  doubleVar is of type double         Or more general  How do I compare two types so that compare double1 double2  will evaluate to true  and compare int double  will evaluate to false  Also I d like to compare structs of different composition as well   Basically  I have a function that operates on variables of type  struct a  and  struct b   I want to do one thing with the  struct a  variables and the other with the  struct b  variables  Since C doesn t support overloading and the void pointer losses its type information I need to check for type  BTW  what would be the sense in having a typeof operator  if you can t compare types     The sizeof method seems to be a practical workaround solution for me  Thanks for your help  I still find it a bit strange since the types are known at compile time  but if I imagine the processes in the machine I can see  why the information is not stored in terms of types  but rather in terms of byte size  Size is the only thing really relevant besides addresses

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From linux typecheck h         Check at compile time that something is of a particular type     Always evaluates to 1 so you may use it easily in comparisons       define typecheck type x        type   dummy        typeof x    dummy2         void   amp   dummy     amp   dummy2         1         Here you can find explanation which statements from standard and which GNU extensions above code uses    Maybe a bit not in scope of the question  since question is not about failure on type mismatch  but anyway  leaving it here

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As other people have already said this isn t supported in the C language  You could however check the size of a variable using the sizeof   function  This may help you determine if two variables can store the same type of data   Before you do that  read the comments below

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Gnu GCC has a builtin function for comparing types   builtin types compatible p   https   gcc gnu org onlinedocs gcc-3 4 5 gcc Other-Builtins html     This built-in function returns 1 if the unqualified versions of the   types type1 and type2  which are types  not expressions  are   compatible  0 otherwise  The result of this built-in function can be   used in integer constant expressions       This built-in function ignores top level qualifiers  e g   const    volatile   For example  int is equivalent to const int    Used in your example   double doubleVar  if   builtin types compatible p typeof doubleVar   double         printf  doubleVar is of type double

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C is statically typed language  You can t declare a function which operate on type A or type B  and you can t declare variable which hold type A or type B  Every variable has an explicitly declared and unchangeable type  and you supposed to use this knowledge   And when you want to know if void   points to memory representation of float or integer - you have to store this information somewhere else  The language is specifically designed not to care if char   points to something stored as int or char

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C does not support this form of type introspection  What you are asking is not possible in C  at least without compiler-specific extensions  it would be possible in C    however    In general  with C you re expected to know the types of your variable  Since every function has concrete types for its parameters  except for varargs  I suppose   you don t need to check in the function body  The only remaining case I can see is in a macro body  and  well  C macros aren t really all that powerful   Further  note that C does not retain any type information into runtime  This means that  even if  hypothetically  there was a type comparison extension  it would only work properly when the types are known at compile time  ie  it wouldn t work to test whether two void   point to the same type of data    As for typeof  First  typeof is a GCC extension  It is not a standard part of C  It s typically used to write macros that only evaluate their arguments once  eg  from the GCC manual      define max a b          typeof  a   a    a           typeof  b   b    b           a  gt   b    a    b       The typeof keyword lets the macro define a local temporary to save the values of its arguments  allowing them to be evaluated only once   In short  C does not support overloading  you ll just have to make a func a struct a    and func b struct b     and call the correct one  Alternately  you could make your own introspection system   struct my header     int type       define TYPE A 0  define TYPE B 1  struct a     struct my header header                  struct b     struct my header header                  void func a struct a  p   void func b struct b  p    void func switch struct my header  head    define func p  func switch   amp  p - gt header    void func switch struct my header  head      switch  head- gt type        case TYPE A  func a  struct a   head   break      case TYPE B  func b  struct b   head   break      default  assert    UNREACHABLE   0             You must  of course  remember to initialize the header properly when creating these objects

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Getting the type of a variable is  as of now  possible in C11 with the  Generic generic selection  It works at compile-time  The syntax is a bit like that for switch  Here s a sample  from this answer        define typename x   Generic  x                                                                  Bool   quot  Bool quot                    unsigned char   quot unsigned char quot                           char   quot char quot                       signed char   quot signed char quot                        short int   quot short int quot           unsigned short int   quot unsigned short int quot                       int   quot int quot                       unsigned int   quot unsigned int quot                        long int   quot long int quot             unsigned long int   quot unsigned long int quot              long long int   quot long long int quot   unsigned long long int   quot unsigned long long int quot                 float   quot float quot                           double   quot double quot                           long double   quot long double quot                     char     quot pointer to char quot                       void     quot pointer to void quot                  int     quot pointer to int quot                       default   quot other quot    To actually use it for compile-time manual type checking  you can define an enum with all of the types you expect  something like this      enum t typename           TYPENAME BOOL          TYPENAME UNSIGNED CHAR          TYPENAME CHAR          TYPENAME SIGNED CHAR          TYPENAME SHORT INT          TYPENAME UNSIGNED CHORT INT          TYPENAME INT                            TYPENAME POINTER TO INT          TYPENAME OTHER         And then use  Generic to match types to this enum       define typename x   Generic  x                                                                        Bool  TYPENAME BOOL            unsigned char  TYPENAME UNSIGNED CHAR                          char  TYPENAME CHAR              signed char  TYPENAME SIGNED CHAR                       short int  TYPENAME SHORT INT  unsigned short int  TYPENAME UNSIGNED SHORT INT                      int  TYPENAME INT                                                                                           int    TYPENAME POINTER TO INT                       default  TYPENAME OTHER

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As others have mentioned  you can t extract the type of a variable at runtime   However  you could construct your own  object  and store the type along with it   Then you would be able to check it at runtime   typedef struct      int  type         or this could be an enumeration    union         double d        int i       u    CheesyObject    Then set the type as needed in the code   CheesyObject o  o type   1      or better as some define  enum value    o u d   3 14159

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For that purpose I have written a simple C program for that    It is in github   GitHub Link  Here how it works    First convert your double into a char string named s    char s 50   sprintf s    2f   yo     Then use my dtype function to determine the type    My function will return a single character   You can use it like this     char type dtype s     Return types are     i for integer   f for float or decimals   c for character      Then you can use comparison to check it    That s it

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As another answer mentioned  you can now do this in C11 with  Generic   For example  here s a macro that will check if some input is compatible with another type    include  lt stdbool h gt   define isCompatible x  type   Generic x  type  true  default  false    You can use the macro like so   double doubleVar  if  isCompatible doubleVar  double         printf  doubleVar is of type double  n        prints    int intVar  if  isCompatible intVar  double         printf  intVar is compatible with double too  n        doesn t print     This can also be used on other types  including structs  E g   struct A       int x      int y      struct B       double a      double b      int main void            struct A AVar    4  2       struct B BVar    4 2  5 6        if  isCompatible AVar  struct A             printf  Works on user-defined types  n        prints            if  isCompatible BVar  struct A             printf  And can differentiate between them too  n        doesn t print            return 0      And on typedefs   typedef char  string   string greeting    Hello world    if  isCompatible greeting  string         printf  Can check typedefs  n        However  it doesn t always give you the answer you expect  For instance  it can t distinguish between an array and a pointer   int intArray      4  -9  42  3    if  isCompatible intArray  int          printf  Treats arrays like pointers  n          The code below doesn t print  even though you d think it would if  isCompatible intArray  int 4          printf  But at least this works  n        Answer borrowed from here  http   www robertgamble net 2012 01 c11-generic-selections html

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This is crazily stupid  but if you use the code   fprintf   x   variable    and you use the -Wall flag while compiling  then gcc will kick out a warning of that it expects an argument of  unsigned int  while the argument is of type           If this warning doesn t appear  then your variable is of type  unsigned int     Best of luck   Edit   As was brought up below  this only applies to compile time   Very helpful when trying to figure out why your pointers aren t behaving  but not very useful if needed during run time

[c] How do I check if a variable is of a certain type (compare two types) in C?

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