2004-11-28 03:35:12 +00:00
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2004-11-28 03:35:12 +00:00
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< title > THE BOOST MPL LIBRARY: Handling Placeholders</ title >
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< td class = "header-group page-location" >< a href = "../index.html" class = "navigation-link" > Front Page</ a > / < a href = "./tutorial-metafunctions.html" class = "navigation-link" > Tutorial: Metafunctions and Higher-Order Metaprogramming</ a > / < a href = "./handling-placeholders.html" class = "navigation-link" > Handling Placeholders</ a ></ td >
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< h1 >< a class = "toc-backref" href = "./tutorial-metafunctions.html#id48" name = "handling-placeholders" > Handling Placeholders</ a ></ h1 >
< p > Our implementation of < tt class = "literal" >< span class = "pre" > twice</ span ></ tt > already works with metafunction
classes. Ideally, we would
like it to work with placeholder expressions too, much the same as
< tt class = "literal" >< span class = "pre" > mpl::transform</ span ></ tt > allows us to pass either form. For example, we
would like to be able to write:</ p >
< pre class = "literal-block" >
template < class X>
struct two_pointers
: twice< < strong > boost::add_pointer< _1> </ strong > , X>
{};
</ pre >
<!-- @ example.append('typedef two_pointers<int>::type intstar2;')
compile('all', pop = 1, expect_error = True) -->
< p > But when we look at the implementation of < tt class = "literal" >< span class = "pre" > boost::add_pointer</ span ></ tt > ,
it becomes clear that the current definition of < tt class = "literal" >< span class = "pre" > twice</ span ></ tt > can't
work that way.</ p >
< pre class = "literal-block" >
template < class T>
struct add_pointer
{
typedef T* type;
};
</ pre >
<!-- @ compile() -->
< p > To be invokable by < tt class = "literal" >< span class = "pre" > twice</ span ></ tt > , < tt class = "literal" >< span class = "pre" > boost::add_pointer< _1> </ span ></ tt > would have
to be a metafunction class, along the lines of < tt class = "literal" >< span class = "pre" > add_pointer_f</ span ></ tt > .
Instead, it's just a nullary metafunction returning the almost
senseless type < tt class = "literal" >< span class = "pre" > _1*</ span ></ tt > . Any attempt to use < tt class = "literal" >< span class = "pre" > two_pointers</ span ></ tt > will
fail when < tt class = "literal" >< span class = "pre" > apply1</ span ></ tt > reaches for a nested < tt class = "literal" >< span class = "pre" > ::apply</ span ></ tt >
metafunction in < tt class = "literal" >< span class = "pre" > boost::add_pointer< _1> </ span ></ tt > and finds that it
doesn't exist.</ p >
< p > We've determined that we don't get the behavior we want
automatically, so what next? Since < tt class = "literal" >< span class = "pre" > mpl::transform</ span ></ tt > can do this
sort of thing, there ought to be a way for us to do it too — and
so there is.</ p >
< ul class = "toc simple" id = "outline" >
< li >< a class = "reference" href = "./the-lambda-metafunction.html" id = "id49" name = "id49" > The < tt class = "literal" >< span class = "pre" > lambda</ span ></ tt > Metafunction</ a ></ li >
< li >< a class = "reference" href = "./the-apply-metafunction.html" id = "id50" name = "id50" > The < tt class = "literal" >< span class = "pre" > apply</ span ></ tt > Metafunction</ a ></ li >
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