[boost] [parameter] - catching accidental duplicate parameters at compile time

From: Daniel Earwicker (daniel.earwicker_at_[hidden])
Date: 2005-11-01 15:34:34

To take the first code snippet from the tutorial:

    new_window("alert", width=10, titlebar=false);

Suppose the user accidentally does this:

    new_window("alert", width=10, titlebar=false, width=2);

In the current version of the library this compiles, and by examining
the source of the library we can discover that one of the values for
'width' will be used and the other discarded (working out which is
left as an exercise for the reader.)

It would be nice if this was caught at compile time - especially if
the parameter "values" are complex expression templates, making it
hard to spot the problem at first glance.

One implementation would be to add a recursive metafunction to the
body of arg_list:

    template<class SAME_AS>
    struct find_duplicates
    {
        enum
        {
            value = (boost::is_same<SAME_AS, key_type>::value ||
                Next::template find_duplicates<SAME_AS>::value)
        };
    };

    BOOST_STATIC_ASSERT(!(Next::template find_duplicates<key_type>::value));

And a terminator to empty_arg_list:

    template<class SAME_AS>
    struct find_duplicates
    {
        enum { value = false };
    };

So an arg_list instantiation will not compile if it adds itself to the
front of a list that already contains an arg_list entry using the same
keyword type.

Hope this is worthwhile.

--Daniel.