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isaacyho@gmail.com · Guest

I seem to have trouble using template policies to achieve the
following, and was wondering if this was a "misuse" of templates ( to
the extent that there are no good ways of doing this ):

I have a class for which it would be great to be able to toggle
various members present / not present in the class. An example would
be a Car:

class Car
{
Engine engine;
Spoiler spoiler;
Radio radio;
};

The engine should always be there, but the spoiler and radio may/may
not. An idea is to use templates:

class HasSpoilerPolicy
{
Spoiler spoiler;
};

class HasAmFmStereoPolicy
{
AmFmStereo stereo; // can be am/fm, or cd player, or none
};
class HasCDStereoPolicy
{
CDStereo stereo; // can be am/fm, or cd player, or none
};

template
< SPOILER_POLICY, RADIO_POLICY >
class Car : public SPOILER_POLICY, RADIO_POLICY
{
Engine engine;
};

The motivation is to only include the data members needed by the
client: if you don't have a radio in the car, don't include even a
pointer for it.

These design decisions do seem orthogonal: the presence of a spoiler
does not affect the engine or radio, for instance. But the problem
seems to be that of coordinating the various classes, esp. during
construction. How do we get a clean construction syntax for such an
object? The best I can come up with is

template
<SPOILER_POLICY, SPOILER_CTOR_T, RADIO_POLICY, RADIO_CTOR_T>
class Car : public SPOILER_POLICY, RADIO_POLICY
{
public:
Car( const Engine & engine_, SPOILER_CTOR_T spoiler_, RADIO_CTOR_T
radio_ ) : SPOILER_POILCY( spoiler_ ), RADIO_POLICY( radio_ ) {}
};

class NullPolicy
{
template <T> NullPolicy( T const & ) {}
};

with client syntax

main()
{
Engine engine;
AmFmRadio radio;

Car< NullPoilcy, HasRadioPoilcy > car( engine, null, radio );
}

This forces the client to include a dummy value in the Car constructor
for the Null fields.

Any better way? Or should I just give up using templates for this
purpose ( toggling central data values )? This solution does seem
very similar to the policies presented by Andresceau ( smart_ptr ),
except that the toggled data structures have to be constructed. It's
nice because it could potentially solve the combinatorial explosion
resulting from 2^n possible toggles, so I'd like to use it if I can do
it in a cleaner way.

Thanks,

Isaac

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Michiel Salters · Guest

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Nicola Musatti · Guest

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Craig Henderson · Guest

<isaacyho (AT) gmail (DOT) com> wrote