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43 lines
2.2 KiB
Text
43 lines
2.2 KiB
Text
The inverse operation of determining the index of a certain type in a
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tt(TypeList) is retrieving the type given its index. This inverse operation is
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the topic of this section.
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The algorithm is implemented using a struct tt(TypeAt). tt(TypeAt) uses a
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tt(typedef) to define the type matching a given index. But the index might be
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out of bounds. In that case we have several options:
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itemization(
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it() Use a link(static_assert)(STATICASSERT) to stop the compilation. This
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is an appropriate action if the index should simply not be out of bounds;
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it() Define a local type (e.g., tt(Null)) that should not be used as a
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type in the tt(TypeList). This type is going to be returned when the index is
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out of bounds. Using this local type as one of the types in a tt(TypeList) is
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considered an error as its would conflict with the special meaning of tt(Null)
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as the type returned at an invalid index.nl()
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To prevent tt(Null) from being returned by tt(TypeAt) a tt(static_assert)
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is used to catch the tt(Null) type when it is encountered while evaluating
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tt(TypeAt);
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it() The struct tt(TypeAt) may define an enum value tt(validIndex) set to
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tt(true) if the index was valid and set to tt(false) if not.
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)
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The first alternative is implemented below. The other alternatives are not
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difficult to implement and are left as exercises for the reader. Here's how
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tt(TypeAt) works:
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itemization(
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it() The foundation consists of a variadic template struct tt(TypeAt),
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expecting an index and a tt(TypeList):
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verbinsert(TYPEAT)(examples/typeat.h)
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it() If the typelist is empty a tt(static_assert) ends the compilation
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verbinsert(INVALID)(examples/typeat.h)
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it() If the search index equals 0, define tt(Type) as the first type in
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the tt(TypeList):
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verbinsert(ZERO)(examples/typeat.h)
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it() Otherwise, tt(Type) is defined as tt(Type) defined by tt(TypeAt<index
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- 1>) operating on the tt(TypeList)'s tail:
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verbinsert(TYPELIST)(examples/typeat.h)
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)
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Here is how tt(typeAt) can be used. Uncommenting the first variable
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definition causes a tt(TypeAt index out of bounds) compilation error:
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verbinsert(EXAMPLE)(examples/typeat.cc)
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