error : expected primary-expression before 'decltype'

Question

I am trying to find the type of variable. In stackoverflow it is mentioned that decltype() is used for that purpose. But when I tried to used it is throwing me the error as I mentioned in title.

#include <bits/stdc++.h>
using namespace std;
int main()
{
    int x = 4;
    cout << decltype(x);
    return 0;
}

I expected int but it showing as error. error: expected primary-expression before 'decltype'

Answer 1

Types aren't first class objects. You can't pass a type to a function, and cout << decltype(x) is exactly that, passing a type to a function (though beautified by the operator).

To get an info about the type of a variable, you can

1. Read the code. If the type of an object is int, don't bother printing it.
2. Step through your program with a debugger. It shows the type of variables.

3. Use this (non-standard) function template

   template <class T> void printType(const T&)
   {
       std::cout << __PRETTY_FUNCTION__ << "\n";
   }
   
   printType(x);
   

4. Use Boost.

   #include <boost/type_index.hpp>
   
   std::cout << boost::typeindex::type_id_with_cvr<decltype(x)>().pretty_name() << "\n";