Re: How to avoid complex switches?

Ian Collins <>
Mon, 29 Mar 2010 14:27:43 +1300
On 03/29/10 09:55 AM, none wrote:

Ian Collins wrote:

On 03/29/10 06:32 AM, none wrote:

I have a class that takes a few template parameters:

template<typename A, typename B, typename C,typename D>
class MyClass {
// ...

The types A,B,C and D are selected from a user specified input file
(properties file):

A = 1
B = 2
C = 1
D = 3

I then parse this file an need to create MyClass with the correct types:

<snip big ugly switch>

But this switch grows extremely large when the number of choices for
each type grows and is also very ugly/error prone. It could be nice if
it was possible to do something like this instead:

Could you use the factory pattern? If you create a polymorphic base
for MyClass, you can have simple factory objects:

struct MyClassFactoryBase
virtual MyClassBase* build() = 0

template<typename A, typename B, typename C,typename D>
struct MyClassFactory : MyClassFactoryBase
MyClass<A,B,C,D>* build() { ... }

You can yen populate a lookup table with MyClassFactory objects.

But I still need to parse the user specified selection into to the
correct types, so I don't see how you can avoid a switch with the above?

By populating the lookup table with the appropriate factory objects.

Something like (reducing the dimensions to 2 for simplicity):

std::vector< std::vector<MyClassFactoryBase*> > lookup(2);

void populateLookup()
   lookup[0][0] = new MyClassFactory<A,B,C,D>;
   lookup[0][1] = new MyClassFactory<D,A,B,C>;
   lookup[1][0] = new MyClassFactory<C,D,A,B>;
   lookup[1][1] = new MyClassFactory<B,C,D,A>;

MyClassBase* createMyClass( int a, int b )
   return lookup[a][b]->build();

Ian Collins

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