Let's say I have a constant value (possibly of some enum type). Let's say I have many classes A, B, D, etc.
Can I have something like this?
C<1> anInstanceOfA; //This will be of type A
C<2> anInstanceOfB; //This will be of type B
C<3> anInstanceOfD; //This will be of type D
So, is it possible to select a class based on a constant number at compile time?
The general problem is that I am trying to select a functor based on a table, in which the index is an enum. I would like to avoid polymorfism if possible.
Edit: For this project I cannot use C++11, thanks anyway to who answered in that context, very interesting to know anyway.
Edit 2: In general I can have more than 2 target classes, I have edited my question
This isn't the only way to do this, but I hope acceptable for your purposes:
struct A { };
struct B { };
template <int N>
struct choices;
template <>
struct choices<1> { typedef A type; };
template <>
struct choices<2> { typedef B type; };
template <int N>
using C = typename choices<N>::type;
Update: To do the same without C++11 features, you should make C
a class with a typedef
member type equal to the corresponding type alias above:
template <int N>
struct C
{
typedef typename choices<N>::type type;
};
// ...
C<1>::type anInstanceOfA;
C<2>::type anInstanceOfB
Using the LSP and plain C++98:
template <int N> class C;
template <> class C<1> : public A {};
template <> class C<2> : public B {};
template <> class C<3> : public D {};
C<1> anInstanceOfA;
Since public inheritance in C++ satisfies the IS-A rule, anInstanceOfA
both IS-A C<1>
object and IS_AN A
object.
This is a fairly simple metafunction:
template <int N>
struct C {
typedef typename std::conditional<N == 1,A,B>::type type;
};
You would use this as C<1>::type foo;
.
If your compiler supports C++11 template aliases, you can simplify to:
template <int N>
using C = typename std::conditional<N == 1,A,B>::type;
and have your preferred C<1> foo;
syntax.
In pure C++03, implement std::conditional
as:
template <bool, typename A, typename>
struct conditional {
typedef A type;
};
template <typename A, typename B>
struct conditional<false, A, B> {
typedef B type;
};
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