How to implement this function?

[28poGNh0]

/// I have this class ... I could not get any way to implement the isNbrOdd function outside the class body
/// Is there a way to do so thanks in advance

class myClass
{
    public :
        template<class T>
        bool isNbrOdd(T nbr)
        {
            if(typeid(T)==typeid(size_t))
            {
                if(nbr%2)
                    return true;
                return false;
            }
            else{
                cout << "You should use the size_t/unsigned int type" << endl;
                return false;
            }
        }
}

;

[MikeyBoy]

No, it's not possible. The template function is instantiated for the specific type of T at compile time. The compiler needs to have the entire function definition visible when it compiles the function call. The entire function definition therefore needs to be in the header file.

[JLBorges]

> I could not get any way to implement the isNbrOdd function outside the class body
> Is there a way to do so thanks in advance

You can define it outside the class body, just as you would for any other member function.

Because it is a template member,
a. define it as a template
b. define it in the header

class myClass
{
    public :
        template<class T> bool isNbrOdd(T nbr) ;
};

template<class T>
bool myClass::isNbrOdd(T nbr)
{
    if(typeid(T)==typeid(size_t))
    {
        if(nbr%2)
            return true;
        return false;
    }
    else{
        cout << "You should use the size_t/unsigned int type" << endl;
        return false;
    }
}

However, typeid(T)==typeid(size_t) is not a particularly good idea; it is (logically) evaluated at runtime.
And therefore, if we try to call myClass::isNbrOdd() with (say) a double, we would get a compile-time error:
nbr%2 is an invalid operation on a double.

A better approach would be to use SFINAE:

#include <iostream>
#include <type_traits>

class myClass
{
    public :
        template < typename  T > inline static
            typename std::enable_if< std::is_unsigned<T>::value, bool >::type
                isNbrOdd( T nbr ) ;

        template < typename  T > inline static
            typename std::enable_if< !std::is_unsigned<T>::value, bool >::type
                isNbrOdd( T nbr ) ;
};

template < typename  T >
    typename std::enable_if< std::is_unsigned<T>::value, bool >::type
        myClass::isNbrOdd( T nbr ) { return nbr%2 ; }

template < typename  T >
    typename std::enable_if< !std::is_unsigned<T>::value, bool >::type
        myClass::isNbrOdd( T )
        {
            std::cout << "please use an unsigned integral type\n" ;
            return false ;
        }

int main()
{
    std::cout << std::boolalpha ;

    std::cout << "unsigned long long: " << myClass::isNbrOdd( 79ULL ) << "\n\n" ;

    std::cout << "double: " << myClass::isNbrOdd( 79.0 ) << "\n\n" ;

    unsigned short s = 79 ;
    std::cout << "unsigned short: " << myClass::isNbrOdd(s) << '\n' ;
}

http://coliru.stacked-crooked.com/a/19230bf96e0878b1

[28poGNh0]

OMG you're so awsome ,I really focused on getting a natural number than I forget to pay attention on double ,thanks a lot .One question though ,what is SFINAE ?

[Zhuge]

Wikipedia actually has an article on it:
https://en.wikipedia.org/wiki/Substitution_failure_is_not_an_error

[28poGNh0]

Thanks @Zhuge

@JLBorges ,I'll remember this name as one of the most talented in this website thanks a lot

[JLBorges]

Talented?
This is nothing more than routine familiarity with a fairly common C++ idiom.

[28poGNh0]

For me you're talented .

To be honest I am still not used to work with templates yet, so what you did above is a hard code for me,also you give me one precise answer on other thread .I like encoutering people who dont have to answer twice.Thats way I called you talented ,talented