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;
};
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
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