Templated Defaulted default constructor

Question

I would like to have the definition of the default constructor be switched at compile time by a template parameter. I can get this to compile OK for a conversion constructor, but trying to use that method for a default constructor to be defaulted or not--useful if in the case of particular template parameter, the resulting class could be POD, but in another case, it could not--but I get a compiler error when doing so. Short of specializing the template and duplicating all the code that's the same, is there a way to do this? Here's a simplified version of what I was trying:

#include<type_traits>   // for enable_if

template <bool MyParameter>
class Demonstration
{
    public:

        //trivial copy, move constructors/assignment, and trivial destructor
        constexpr Demonstration(Demonstration const &) = default;
        constexpr Demonstration(Demonstration &&) = default;
        Demonstration & operator= (Demonstration const &) = default;
        Demonstration & operator= (Demonstration &&) = default;
        ~Demonstration() = default;

        // this one gives "error: a template cannot be defauled"
        template <bool Dummy=MyParameter, typename std::enable_if< Dummy , bool >::type=true >
        Demonstration() = default;

        // ok
        template <bool Dummy=MyParameter, typename std::enable_if< !Dummy , bool >::type=false >
        Demonstration() : myValue(0) {}

        // ok
        template <bool Dummy=MyParameter, typename std::enable_if< Dummy , bool >::type=true >
        explicit constexpr Demonstration(unsigned char toConvert)
        : myValue ( toConvert )
        {
        }
        // ok
        template <bool Dummy=MyParameter, typename std::enable_if< !Dummy , bool >::type=false >
        explicit constexpr Demonstration(unsigned char toConvert)
        : myValue ( toConvert > 100 ? 0 : toConvert )
        {
        }

    // a lot of functions that do not depend on parameter go here

    protected:
    private:
        unsigned char myValue;

};

Answer 1

GCC complains of your template:

error: a template cannot be defaulted

and Clang complains:

error: only special member functions may be defaulted.

That seems fair enough. A member function template is not a member function, let alone a special one.

You would like Demonstration<bool P> to be POD when P is true and otherwise not necessarily so.

A possible solution is to delegate the parameterization of POD-ness entirely to specializations of a base template base<bool P> and have Demonstration<P> inherit base<P>. Here is an illustration:

#include<type_traits>

template<bool Param = true>
struct base // is POD 
{
    base() = default;
    explicit constexpr base(unsigned char ch)
    : _val(ch){}
    unsigned char _val;
};

template<>
struct base<false> // is not POD
{
    base() = default;
    explicit constexpr base(unsigned char ch)
    : _val(ch > 100 ? 0 : ch){}
    unsigned char _val = 0;
};


template <bool MyParameter>
class Demonstration : private base<MyParameter>
{   
public:

    Demonstration() = default;
    //trivial copy, move constructors/assignment, and trivial destructor
    constexpr Demonstration(Demonstration const &) = default;
    constexpr Demonstration(Demonstration &&) = default;
    Demonstration & operator= (Demonstration const &) = default;
    Demonstration & operator= (Demonstration &&) = default;
    ~Demonstration() = default;

    explicit constexpr Demonstration(unsigned char toConvert)
    : base<MyParameter>(toConvert)
    {
    }

    char myValue() const {
        return base<MyParameter>::_val;
    }
};


#include <iostream>

using namespace std;

int main()
{
    cout << "is_pod<base<true>>::value = " 
        << is_pod<Demonstration<true>>::value << endl;
    cout << "is_pod<base<false>>::value = " 
        << is_pod<Demonstration<false>>::value << endl;
    cout << "is_pod<Demonstration<true>>::value = " 
        << is_pod<Demonstration<true>>::value << endl;
    cout << "is_pod<Demonstration<false>>::value = " 
        << is_pod<Demonstration<false>>::value << endl;
    Demonstration<true> d_true(1);
    Demonstration<false> d_false(101);
    std::cout << "(int)Demonstration<true>(1).myValue() = " 
        << (int)d_true.myValue() << endl;
    std::cout << "(int)Demonstration<false>(101).myValue() = " 
        << (int)d_false.myValue() << endl;
    return 0;
}

Now Demonstration<P> is POD just in case base<P> is POD. The program outputs:

is_pod<base<true>>::value = 1
is_pod<base<false>>::value = 0
is_pod<Demonstration<true>>::value = 1
is_pod<Demonstration<false>>::value = 0
(int)Demonstration<true>(1).myValue() = 1
(int)Demonstration<false>(101).myValue() = 0

Built with GCC 4.8.2 and clang 3.3