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## Iterator Concepts

### Access

#### Readable Iterator Concept

A class or built-in type `X` models the Readable Iterator concept for value type `T` if, in addition to `X` being Assignable and Copy Constructible, the following expressions are valid and respect the stated semantics. `U` is the type of any specified member of type `T`.

Table 1.1. Readable Iterator Requirements (in addition to Assignable and Copy Constructible)

Expression | Return Type | Note/Precondition
--- | --- | ---
`iterator_traits<X>::value_type` | `T` | Any non-reference, non cv-qualified type
`*a` | Convertible to `T` | pre: `a` is dereferenceable. If `a == b` then `*a` is equivalent to `*b`.
`a->m` | `U&` | pre: `(*a).m` is well-defined. Equivalent to `(*a).m`.

#### Writable Iterator Concept

A class or built-in type `X` models the Writable Iterator concept if, in addition to `X` being Copy Constructible, the following expressions are valid and respect the stated semantics. Writable Iterators have an associated set of value types.

Table 1.2. Writable Iterator Requirements (in addition to Copy Constructible)

Expression | Return Type | Precondition
--- | --- | ---
`*a = o` |  | pre: The type of `o` is in the set of value types of `X`

#### Swappable Iterator Concept

A class or built-in type `X` models the Swappable Iterator concept if, in addition to `X` being Copy Constructible, the following expressions are valid and respect the stated semantics.

Table 1.3. Swappable Iterator Requirements (in addition to Copy Constructible)

Expression | Return Type | Postcondition
--- | --- | ---
`iter_swap(a, b)` | `void` | the pointed to values are exchanged

Note: An iterator that is a model of the Readable and Writable Iterator concepts is also a model of Swappable Iterator. --end note

#### Lvalue Iterator Concept

The Lvalue Iterator concept adds the requirement that the return type of `operator*` type be a reference to the value type of the iterator.

Table 1.4. Lvalue Iterator Requirements

Expression | Return Type | Note/Assertion
--- | --- | ---
`*a` | `T&` | `T` is cv `iterator_traits<X>::value_type` where cv is an optional cv-qualification. pre: `a` is dereferenceable. If `a == b` then `*a` is equivalent to `*b`.