[Boost-users] Re: [Foreach]TroubleiteratingoverBoost.MultiIndexcontainers

David Abrahams wrote:

Ian McCulloch writes:

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David Abrahams wrote:

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Ian McCulloch writes:

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* Coupling; you need to include a primary template.

Ok, but what does that primary template do?  Surely nothing, or some very conservative default.

Actually you don't need to include a primary template. All you need is a declaration with no body. You can do that without even pulling in a header.

I think, an empty body is better than a bare declaration. At least, with

template <typename T> struct my_algo_impl {};

you can do SFINAE on whether my_algo_impl<T> is actually defined or not, for some T (ie. using T::result_type, or probably better boost::result_of).

That's a useful technique, but I don't see how it relates to coupling. In order for a user to provide a specialization of some library template for her type, she need only provide the bare declaration. Bare declarations and empty definitions aren't alternatives, they're apples and oranges.

True. I had forgotten that you might want to define a specialization but only make use of it elsewhere. But what happens if the primary template has some additional paramters with default values (eg, Enable = void)? Would you regard that as bad style ? Actually, I tend to avoid that anyway, and instead if the primary template has a default then forward it to another template that does the enable_if tricks/hacks. But thats mostly because I usually end up very confused when I try to combine partial specialization and enable_if ;-) Cheers, Ian