How do I get the type of a variable

Question

In C    how does one find the type of a variable

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Usually  wanting to find the type of a variable in C   is the wrong question  It tends to be something you carry along from procedural languages like for instance C or Pascal    If you want to code different behaviours depending on type  try to learn about e g  function overloading and object inheritance  This won t make immediate sense on your first day of C    but keep at it

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I m not sure if my answer would help   The short answer is  you don t really need want to know the type of a variable to use it   If you need to give a type to a static variable  then you may simply use auto   In more sophisticated case where you want to use  auto  in a class or struct  I would suggest use template with decltype   For example  say you are using someone else s library and it has a variable called  unknown var  and you would want to put it in a vector or struct  you can totally do this   template  lt typename T gt  struct my struct       int some field      T my data     vector lt decltype unknown var  gt  complex vector  vector lt my struct lt decltype unknown var  gt   gt  simple vector   Hope this helps   EDIT  For good measure  here is the most complex case that I can think of  having a global variable of unknown type  In this case you would need c  14 and template variable   Something like this   template lt typename T gt  vector lt T gt  global var   void random func  auto unknown var        global var lt decltype unknown var  gt  push back unknown var       It s still a bit tedious but it s as close as you can get to typeless languages  Just make sure whenever you reference template variable  always put the template specification there

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You can use the typeid operator    include  lt typeinfo gt      cout  lt  lt  typeid variable  name    lt  lt  endl

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You can definitely go for typeid x  name   where x is the variable name  It actually returns a const char pointer to the data type  Now  look at the following code    include lt bits stdc   h gt  using namespace std  int main         int n   36      char c    A       double d   1 2      if   typeid n  name        i            cout  lt  lt   I am an Integer variable   lt  lt  endl            if    char    typeid d  name        d            cout  lt  lt   I am a Double variable   lt  lt  endl            if    char    typeid c  name        c            cout  lt  lt   I am a Char variable   lt  lt  endl            return 0      Notice how first and second both if works

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If you need to make a comparison between a class and a known type  for example   class Example        Example eg   Example      You can use this comparison line   bool isType   string  typeid eg  name     find  Example      string  npos    which checks the typeid name contains the string type  the typeid name has other mangled data  so its best to do a s1 find s2  instead of

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For static assertions  C  11 introduced decltype which is quite useful in certain scenarios

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I believe I have a valid use case for using typeid    the same way it is valid to use sizeof    For a template function  I need to special case the code based on the template variable  so that I offer maximum functionality and flexibility   It is much more compact and maintainable than using polymorphism  to create one instance of the function for each type supported  Even in that case I might use this trick to write the body of the function only once   Note that because the code uses templates  the switch statement below should resolve statically into only one code block  optimizing away all the false cases  AFAIK   Consider this example  where we may need to handle a conversion if T is one type vs another  I use it for class specialization to access hardware where the hardware will use either myClassA or myClassB type  On a mismatch  I need to spend time converting the data   switch   typeid T       case typeid myClassA          handle that case     break    case typeid myClassB          handle that case     break    case typeid uint32 t          handle that case     break    default         handle that case

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The main difference between C   and Javascript is that C   is a static-typed language  wile javascript is dynamic   In dynamic typed languages a variable can contain whatever thing  and its type is given by the value it holds  moment by moment  In static typed languages the type of a variable is declared  and cannot change   There can be dynamic dispatch and object composition and subtyping  inheritance and virtual functions  as well as static-dispatch and supertyping  via template CRTP   but in any case the type of the variable must be known to the compiler   If you are in the position to don t know what it is or could be  it is because you designed something as the language has a dynamic type-system   If that s the case you had better to re-think your design  since it is going into a land not natural for the language you are using  most like going in a motorway with a caterpillar  or in the water with a car

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If you have a variable   int k    You can get its type using   cout  lt  lt  typeid k  name    lt  lt  endl    See the following thread on SO  Similar question

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include  lt typeinfo gt       string s   typeid YourClass  name

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