Checking whether a template argument is of a class type?

Question

How to check using some template hack whether a template argument passed is of class type?

Example

int main()
{
   CheckIfClass<int>::checkConst ; No it is not of a class type
   class CLASS{};
   CheckIfClass<CLASS>::checkConst ; Yes CLASS is a class.
   CheckIfClass<std::string>::checkConst ; Yes std::string is a class
}

Why do you need to know? – Karl Knechtel Dec 04 '10 at 06:29 — Karl Knechtel, Dec 04 '10 at 06:29

Prasoon Saurav · Accepted Answer · 2010-12-04T06:53:50.200

7

SFINAE should do your job

#include <iostream>
template<typename T>
struct Check_If_T_Is_Class_Type
{
    template<typename C> static char func (char C::*p);
    template<typename C> static int func (...);
    enum{val = sizeof (Check_If_T_Is_Class_Type<T>::template func<T>(0)) == 1};
};
class empty{}; // Defined the class in the global namespace. 
               // You can't have local classes as template arguments in C++03

int main()
{

    std::cout<<Check_If_T_Is_Class_Type<empty>::val; // 1
    std::cout<<Check_If_T_Is_Class_Type<int>::val; // 0
    std::cout<<Check_If_T_Is_Class_Type<std::string>::val; //1
}

Output

edited Dec 04 '10 at 06:53

answered Dec 04 '10 at 06:34

Prasoon Saurav

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@UpVoter : Oh ideone compiled the code in c++0x mode. Updated my answer (for C++03). – Prasoon Saurav Dec 04 '10 at 06:46
Wow! I need to figure out how the code is actually working. thanks. – NEWBIE Dec 04 '10 at 06:51
@Nawaz : Yes it is optional :-) – Prasoon Saurav Dec 04 '10 at 07:36
@Prasoon Saurav... your code doesn't compile with MSVC++2008. What's wrong with it? `'char Check_If_T_Is_Class_Type::func(char C::* )' : could not deduce template argument for 'char C::* ' from 'int' 'int Check_If_T_Is_Class_Type::func(...)' : could not deduce template argument for 'C'` – Nawaz Dec 04 '10 at 10:32
@Nawaz : The code gets compiled on Comeau, g++ and Clang. MSVC++ as we all know is buggy. You can create another thread asking for an explanation why this doesn't get compiled on MSVC++. There are so many MS guys out there. :-) – Prasoon Saurav Dec 04 '10 at 10:41
@Nawaz: It would seem to compile if you remove `Check_If_T_Is_Class_Type::template` (should be superfluous anyway?) – UncleBens Dec 04 '10 at 10:44
@UncleBens... yeah I was trying to do those sort of things and eventually discovered it gets compiled if I remove `template` ketyword. I've posted the solution! – Nawaz Dec 04 '10 at 11:58

score 2 · Answer 2 · answered Dec 04 '10 at 08:51

2

C++0x offers a very simple solution:

#include <iostream>
#include <type_traits>

int main()
{
    std::cout << is_class< your_type_here >::value << std::endl;
}

answered Dec 04 '10 at 08:51

fredoverflow

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Nawaz · Answer 3 · 2010-12-04T12:14:18.513

Code that compiles with MSVC++08 ALSO, as well as with GCC, Comeau, and Clang (edited).

#include <iostream>
template<typename T>
struct Check_If_T_Is_Class_Type
{
    template<typename C> static char func (char C::*p);
    template<typename C> static int func (...);
    enum{val = sizeof (func<T>(0)) == 1};
};
class empty{};
int main()
{   
    std::cout<<Check_If_T_Is_Class_Type<empty>::val; // 1
    std::cout<<Check_If_T_Is_Class_Type<int>::val; // 0
    std::cout<<Check_If_T_Is_Class_Type<std::string>::val; //1
}

@Prasoon... could you please compile this on Comeau, and Clang ... and tell me if it gets compiled or not? thanks!

Yes it gets compiled on those :-) – Prasoon Saurav Dec 04 '10 at 12:03 — Prasoon Saurav, Dec 04 '10 at 12:03

Checking whether a template argument is of a class type?

3 Answers3

Code that compiles with MSVC++08 ALSO, as well as with GCC, Comeau, and Clang (edited).

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