Specifies that the type is a pointer-like object which can be compared to `std::nullptr_t`

objects.

The type must meet all of the following requirements:

In addition, a value-initialized object of the type must produce a null value of that type. This null value shall only be equivalent to itself. Default initialization of the type may have an indeterminate value.

An object of the type must be contextually convertible to bool. The effect of this conversion returns false if the value is equivalent to its null value and true otherwise.

None of the operations that this type performs may throw exceptions.

The type must satisfy the following additional expressions, given two values `p`

and `q`

that are of the type, and that `np`

is a value of `std::nullptr_t`

type (possibly `const`

qualified):

Expression | Effects |
---|---|

`Type p(np);`
| Afterwards, `p` is equivalent to `nullptr` |

`Type(np)` | a temporary object that is equivalent to `nullptr` |

`p = np` | Must return a `Type&` , and afterwards, `p` is equivalent to `nullptr` |

`p != q` | Must return a value that is contextually convertible to bool. The effect is |

`p == np`
| Must return a value that is contextually convertible to bool. The effect is |

`p != np`
| Must return a value that is contextually convertible to bool. The effect is |

Note that dereferencing (`operator*`

or `operator->`

) is not required of a NullablePointer type. A minimalistic type that satisfies these requirements is.

class handle { int id; public: handle(std::nullptr_t = nullptr) : id(0) { } explicit operator bool() { return id != 0; } friend bool operator ==(handle l, handle r) { return l.id == r.id; } friend bool operator !=(handle l, handle r) { return !(l == r); } };

The following types must satisfy NullablePointer:

- The member types
`X::pointer`

,`X::const_pointer`

,`X::void_pointer`

and`X::const_void_pointer`

of every Allocator type`X`

- The member type
`X::pointer`

of`std::unique_ptr`

- The type
`std::exception_ptr`

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