C++ template instantiation: Avoiding long switches

Question

I have a class depending on an integer template parameter. At one point in my program I want to use one instantiation of this template, depending on a value of this paramet

Answer 1

arg_int is a runtime parameter so there is no way to attach it directly to a template parameter. You could use some kind of handler table which would remove the switch statement here.

You'd use something like lookup_handler( int N ) returning a type handler which might be a lambda invoking one of those template functions.

Registering all your lambdas on the table could be done recursively starting with the highest numbered one you allow.

template< unsigned N > register_lambda()
{
     table.add( Wrapper() );
     register_lambda< N-1 >;
}

and specialise for register_lambda<0>

Then somewhere you call register_lambda<32> say and you have registered all the numbers from 0 to 32.

One way to implement such a table is:

class lambda_table
{
 typedef std::function lambda_type; 
    public:
        void add( lambda_type );
        bool lookup( size_t key, lambda_type & lambda ) const;
};

From main() or wherever you want to invoke it you have a reference to this table (call it table) then call

lambda_type lambda;
if( table.find( arg_int, lambda ) )
        lanbda();
else
      default_handler();

You might change this to give the table itself a default handler where none has been supplied for this number.

Although lambdas can wrap all kinds of data members you might actually want your templates to be classes in a hierarchy rather than lambdas given the data storage within them.