Identifying primitive types in templates

Question

I am looking for a way to identify primitives types in a template class definition.

I mean, having this class :

template
class A{
void         


        

Answer 1

It cannot be done exactly the way you asked. Here is how it can be done :

template<class T>
class A{
void doWork(){
   bool isPrimitive = boost::is_fundamental<T>::value;
   if(isPrimitive)
     doSomething();
   else
     doSomethingElse(); 
}
private:
T *t; 
};

You may get a warning if you put the value of isPrimitive directly inside the if statement. This is why i introduced a temporary variable.

Answer 2

Yet another similar examples:

#include  <boost/type_traits/is_fundamental.hpp>
#include <iostream>

template<typename T, bool=true>
struct foo_impl
{
    void do_work()
    {
        std::cout << "0" << std::endl;
    }
};
template<typename T>
struct foo_impl<T,false>
{
    void do_work()
    {
        std::cout << "1" << std::endl;
    }
};

template<class T>
struct foo
{
    void do_work()
    {
        foo_impl<T, boost::is_fundamental<T>::value>().do_work();
    }
};


int main()
{
    foo<int> a; a.do_work();
    foo<std::string> b; b.do_work();
}

Answer 3

Boost TypeTraits has plenty of stuff.

Answer 4

Assuming by 'Primitive Type' you mean the built in types you can do a series of template specializations. Your code would become:

template<class T>
struct A{
    void doWork();
private:
    T *t; 
};

template<> void A<float>::doWork()
{
    doSomething();
}

template<> void A<int>::doWork()
{
    doSomething();
}

// etc. for whatever types you like

template<class T> void A<T>::doWork()
{
    doSomethingElse();
}

Answer 5

There is a better way - using SFINAE. With SFINAE you don't have to list out every primitive type. SFINAE is a technique that depends on the idea that when template specialization fails it falls back to a more general template. ( it stands for "Specialization failure is not an error" ).

Also you don't really define if you consider a pointer to be a primitive type, so I'll make you templates for all the combinations.

// takes a pointer type and returns the base type for the pointer.
// Non-pointer types evaluate to void.
template < typename T > struct DePtr                       { typedef void R; }; 
template < typename T > struct DePtr< T * >                { typedef T R; };
template < typename T > struct DePtr< T * const >          { typedef T R; };
template < typename T > struct DePtr< T * volatile >       { typedef T R; };
template < typename T > struct DePtr< T * const volatile > { typedef T R; };

// ::value == true if T is a pointer type
template < class T > struct IsPointer                        { enum { value = false }; };
template < class T > struct IsPointer < T *                > { enum { value = true };  };
template < class T > struct IsPointer < T * const          > { enum { value = true };  };
template < class T > struct IsPointer < T * volatile       > { enum { value = true };  };
template < class T > struct IsPointer < T * const volatile > { enum { value = true };  };

// ::value == true if T is a class type. ( class pointer == false )
template < class T > struct IsClass 
{
   typedef u8 yes; typedef u16 no; 
   template < class C >    static yes isClass( int C::* );
   template < typename C > static no  isClass( ... );
   enum { value = sizeof( isClass<T>( 0 )) == sizeof( yes ) };
};

// ::value == true if T* is a class type. ( class == false )
template < class T > struct IsClassPtr 
{
   typedef u8 yes; typedef u16 no; 
   template < class C >    static yes isClass( int C::* );
   template < typename C > static no  isClass( ... );
   enum { value = sizeof( isClass< typename DePtr< T >::R >( 0 )) == sizeof( yes ) };
};

// ::value == true if T is a class or any pointer type - including class and non-class pointers.
template < class T > struct IsClassOrPtr : public IsClass<T> { };
template < class T > struct IsClassOrPtr < T *                > { enum { value = true };  };
template < class T > struct IsClassOrPtr < T * const          > { enum { value = true };  };
template < class T > struct IsClassOrPtr < T * volatile       > { enum { value = true };  };
template < class T > struct IsClassOrPtr < T * const volatile > { enum { value = true };  };


template < class T > struct IsClassOrClassPtr : public IsClass<T> { };
template < class T > struct IsClassOrClassPtr < T *                > : public IsClassPtr< T*                > { };
template < class T > struct IsClassOrClassPtr < T * const          > : public IsClassPtr< T* const          > { };
template < class T > struct IsClassOrClassPtr < T * volatile       > : public IsClassPtr< T* volatile       > { };
template < class T > struct IsClassOrClassPtr < T * const volatile > : public IsClassPtr< T* const volatile > { };

Answer 6

I guess this can do the job quite nicely, without multiple specializations:

# include <iostream>
# include <type_traits>

template <class T>
inline bool isPrimitiveType(const T& data) {
    return std::is_fundamental<T>::value;
}

struct Foo {
    int x;
    char y;
    unsigned long long z;
};


int main() {

    Foo data;

    std::cout << "isPrimitiveType(Foo): " << std::boolalpha
        << isPrimitiveType(data) << std::endl;
    std::cout << "isPrimitiveType(int): " << std::boolalpha
        << isPrimitiveType(data.x) << std::endl;
    std::cout << "isPrimitiveType(char): " << std::boolalpha
        << isPrimitiveType(data.y) << std::endl;
    std::cout << "isPrimitiveType(unsigned long long): " << std::boolalpha
        << isPrimitiveType(data.z) << std::endl;

}

And the output is:

isPrimitiveType(Foo): false  
isPrimitiveType(int): true  
isPrimitiveType(char): true  
isPrimitiveType(unsigned long long): true