How to create an array of classes types? - c++

Question

I have a single class "Base", and a few tens of classes derived from Base. I would like to have a method that creates me the right class by an index. Like this:
class Base
{
};
class A : public Base
{
}
class B : public Base
{
}
class C : public Base
{
}
Type array = { A, B, C };
and then I could do new array[i];
How could this be achieved with C++(0x)? Usually I would use an the Abstract Factory Pattern. But since I have a LOT of derived classes, this would really slow down the program.
Since the derived classes will be used only once I also taught to use this:
Base *array = { new A, new B, new C };
But this would lead to huge memory consumption, not counting that not every class will always be used.
Any suggestion?

Answer 1

You cannot use an array of classes, but you can use an array of pointers to functions.
typedef std::unique_ptr<Base> (*Creator)();
template <typename T>
std::unique_ptr<Base> make() { return new T{}; }
Creator const array[] = { make<A>, make<B>, make<C> };
int main() {
std::unique_ptr<Base> b = array[1]();
b->foo();
}
For those worried by the cost of creating so many template functions, here is an example:
#include <stdio.h>
struct Base { virtual void foo() const = 0; };
struct A: Base { void foo() const { printf("A"); } };
struct B: Base { void foo() const { printf("B"); } };
struct C: Base { void foo() const { printf("C"); } };
typedef Base* (*Creator)();
template <typename T>
static Base* make() { return new T{}; }
static Creator const array[] = { make<A>, make<B>, make<C> };
Base* select_array(int i) {
return array[i]();
}
Base* select_switch(int i) {
switch(i) {
case 0: return make<A>();
case 1: return make<B>();
case 2: return make<C>();
default: return 0;
}
}
LLVM/Clang generates the following output:
define %struct.Base* #select_array(int)(i32 %i) uwtable {
%1 = sext i32 %i to i64
%2 = getelementptr inbounds [3 x %struct.Base* ()*]* #array, i64 0, i64 %1
%3 = load %struct.Base* ()** %2, align 8, !tbaa !0
%4 = tail call %struct.Base* %3()
ret %struct.Base* %4
}
define noalias %struct.Base* #select_switch(int)(i32 %i) uwtable {
switch i32 %i, label %13 [
i32 0, label %1
i32 1, label %5
i32 2, label %9
]
; <label>:1 ; preds = %0
%2 = tail call noalias i8* #operator new(unsigned long)(i64 8)
%3 = bitcast i8* %2 to i32 (...)***
store i32 (...)** bitcast (i8** getelementptr inbounds ([3 x i8*]* #vtable for A, i64 0, i64 2) to i32 (...)**), i32 (...)*** %3, align 8
%4 = bitcast i8* %2 to %struct.Base*
br label %13
; <label>:5 ; preds = %0
%6 = tail call noalias i8* #operator new(unsigned long)(i64 8)
%7 = bitcast i8* %6 to i32 (...)***
store i32 (...)** bitcast (i8** getelementptr inbounds ([3 x i8*]* #vtable for B, i64 0, i64 2) to i32 (...)**), i32 (...)*** %7, align 8
%8 = bitcast i8* %6 to %struct.Base*
br label %13
; <label>:9 ; preds = %0
%10 = tail call noalias i8* #operator new(unsigned long)(i64 8)
%11 = bitcast i8* %10 to i32 (...)***
store i32 (...)** bitcast (i8** getelementptr inbounds ([3 x i8*]* #vtable for C, i64 0, i64 2) to i32 (...)**), i32 (...)*** %11, align 8
%12 = bitcast i8* %10 to %struct.Base*
br label %13
; <label>:13 ; preds = %9, %5, %1, %0
%.0 = phi %struct.Base* [ %12, %9 ], [ %8, %5 ], [ %4, %1 ], [ null, %0 ]
ret %struct.Base* %.0
}
Unfortunately, it is not quite intelligent enough to automatically inline the functions with a regular array code (known issue with the LLVM optimizer, I don't know if gcc does better)... but using switch it is indeed possible.

Answer 2

typedef Base* BaseMaker();
template <class X> Base* make() {
return new X;
}
BaseMaker* makers[] = { make<A>, make<B>, make<C> };
Base* b = makers[2]();