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### 1078. DE-17: Remove class type_index

Section: 20.17 [type.index] Status: NAD Concepts Submitter: Doug Gregor Opened: 2009-03-20 Last modified: 2016-01-28

Priority: Not Prioritized

View all other issues in [type.index].

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Discussion:

Addresses DE 17

DE-17:

The class `type_index` should be removed; it provides no additional functionality beyond providing appropriate concept maps.

[ 2009-03-31 Peter adds: ]

> It is not true, in principle, that `std::type_index` provides no utility compared to bare `std::type_info*`.

> `std::type_index` can avoid the lifetime issues with `type_info` when the DLL that has produced the `type_info` object is unloaded. A raw `type_info*` does not, and cannot, provide any protection in this case. A `type_index` can (if the implementor so chooses) because it can wrap a smart (counted or even cloning) pointer to the `type_info` data that is needed for `name()` and `before()` to work.

Proposed resolution:

Modify the header <typeinfo> synopsis in 17.7 [support.rtti]p1 as follows:

>     namespace std {
>       class type_info;
>       class type_index;
>       template <class T> struct hash;
>       template<> struct hash<type_indexconst type_info *> : public std::unary_function<type_indexconst type_info *, size_t> {
>         size_t operator()(type_indexconst type_info * indext) const;
>       };
>       concept_map LessThanComparable<const type_info *> see below
>       class bad_cast;
>       class bad_typeid;
>     }

Add the following new subsection

> 18.7.1.1 Template specialization `hash<const type_info *>` [type.info.hash]

>     size_t operator()(const type_info *x) const;

> 1. Returns: `x->hash_code()`

Add the following new subsection

> 18.7.1.2 `type_info` concept map [type.info.concepts]

>     concept_map LessThanComparable<const type_info *> {
>       bool operator<(const type_info *x, const type_info *y) { return x->before(*y); }
>       bool operator<=(const type_info *x, const type_info *y) { return !y->before(*x); }
>       bool operator>(const type_info *x, const type_info *y) { return y->before(*x); }
>       bool operator>=(const type_info *x, const type_info *y) { return !x->before(*y); }
>     }

> 1. Note: provides a well-defined ordering among `type_info const` pointers, which makes such pointers usable in associative containers (23.4).

Remove section 20.17 [type.index]