Check if object is instance of class with template - c++

Question

My class:
template < typename T >
Array<T>{};
(Source data is stored in vector)
I have an object:
Array< string > a;
a.add("test");
And I have an object:
Array< Array< string > > b;
b.add(a);
How can I check:
Is b[0] an instance of Array (regardless of template type)?
Is a[0] an instance of any type except Array?

Answer 1

If you can use C++11, creating your type traits; by example
#include <string>
#include <vector>
#include <iostream>
#include <type_traits>
template <typename T>
struct Array
{
std::vector<T> v;
void add (T const t)
{ v.push_back(t); }
};
template <typename>
struct isArray : public std::false_type
{ };
template <typename T>
struct isArray<Array<T>> : public std::true_type
{ };
template <typename T>
constexpr bool isArrayFunc (T const &)
{ return isArray<T>::value; }
int main()
{
Array<std::string> a;
Array<Array<std::string>> b;
a.add("test");
b.add(a);
std::cout << isArrayFunc(a.v[0]) << std::endl; // print 0
std::cout << isArrayFunc(b.v[0]) << std::endl; // print 1
}
If you can't use C++11 or newer but only C++98, you can simply write isArray as follows
template <typename>
struct isArray
{ static const bool value = false; };
template <typename T>
struct isArray< Array<T> >
{ static const bool value = true; };
and avoid the inclusion of type_traits
--- EDIT ---
Modified (transformed in constexpr) isArrayFunc(), as suggested by Kerrek SB (thanks!).

Answer 2

Below is a shorter version of the solution proposed by max66 that no longer uses struct isArray.
It works in C++98 and later revisions.
#include <string>
#include <vector>
#include <iostream>
template <typename T>
struct Array
{
std::vector<T> v;
void add (T const t)
{ v.push_back(t); }
};
template <typename T>
constexpr bool isArrayFunc (T const &)
{ return false; }
template <typename T>
constexpr bool isArrayFunc (Array<T> const &)
{ return true; }
int main()
{
Array<std::string> a;
Array<Array<std::string>> b;
a.add("test");
b.add(a);
std::cout << isArrayFunc(a.v[0]) << std::endl; // print 0
std::cout << isArrayFunc(b.v[0]) << std::endl; // print 1
}

Answer 3

in c++ you can use
if(typeid(obj1)==typeid(ob2))//or typeid(obj1)==classname
cout <<"obj1 is instance of yourclassname"
in your case you can check that with typeid(obj1)==std::array