3

In specializing a class template, I would like to have one specialization target full-blown classes (complete with constructor, destructor, etc.) and one specialization target primitives (int, float, etc.). The only partial specialization I've seen is with targeting pointer types (via T*). Is there a way to do this?

TheBuzzSaw
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  • What is special about the full-blown classes that you want to specialize differently? Do they have some special member that you need? – Vaughn Cato Jul 02 '12 at 01:50
  • Well, for primitives, I can do a blanket `memcpy` for certain operations, whereas for classes, I need to invoke each copy operation individually. – TheBuzzSaw Jul 02 '12 at 02:03
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    @TheBuzzSaw: Then you don't want to specialize on primitive/classes, but rather on POD vs non-POD types – David Rodríguez - dribeas Jul 02 '12 at 03:51
  • You probably want to specialize based on something like the type trait is_trivially_copyable – bames53 Jul 02 '12 at 04:20
  • @DavidRodríguez-dribeas: And in C++11, the decided factor is `is_standard_layout`. Standard layout classes can be treated as a sequence of bytes, POD being a subset of those. – GManNickG Jul 02 '12 at 05:37

2 Answers2

7

You can used C++11 type_traits. Here is something to get you started, you can specialize more as needed:

#include <type_traits>
#include <iostream>

template<typename T, typename E = void>
struct A; // undefined

template<typename T>
struct A<T, typename std::enable_if<std::is_class<T>::value && !std::is_pod<T>::value>::type> {
        A() { std::cout << "I'm a class, but I'm not a pod type" << std::endl; }
};

template<typename T>
struct A<T, typename std::enable_if<std::is_class<T>::value && std::is_pod<T>::value>::type> {
        A() { std::cout << "I'm a class and a pod type" << std::endl; }
};

template<typename T>
struct A<T, typename std::enable_if<!std::is_class<T>::value>::type> {
        A() { std::cout << "I'm not a class" << std::endl; }
};

class X {};
class Y { ~Y(){} };

int main()
{
        A<X> a1;
        A<Y> a2;
        A<int> a3;
}
Jesse Good
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0

boost::has_trivial_assign should give you this info.

template <class T>
  struct has_trivial_assign : public true_type-or-false_type {};

http://www.boost.org/doc/libs/1_50_0/libs/type_traits/doc/html/boost_typetraits/reference/has_trivial_assign.html

Tom Kerr
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