[#bsl-pair-0b]
= xref:bsl.adoc[bsl]::pair
:relfileprefix: ../
:mrdocs:

A pair of public data members, `first` and `second`, that supports `bslma` allocators.

== Synopsis

Declared in `<bslstl_pair.h>`

[source,cpp,subs="verbatim,replacements,macros,-callouts"]
----
template<
    class T1,
    class T2>
class pair
    : public xref:bsl/Pair_First-00.adoc[Pair_First<T1>]
    , public xref:bsl/Pair_Second-05e.adoc[Pair_Second<T2>]
----

== Description

The class template `pair` provides a pair of public data members, `first` and `second`, of type `T1` and `T2` respectively. If either `T1` or `T2` uses `bslma::Allocator` for memory management, then provide an optional `bslma::Allocator` argument for each constructor, to be passed through to the constructors of `first` and/or `second` as appropriate. The interface to this class is identical to the standard `std::pair` except for the addition of the allocators. Note that the implementation of this class provides `first` and `second` through multiple base classes in order to simplify construction of each member when allowing for the various rules for passing allocators in C++11.

== Base Classes

[cols="1,4"]
|===
| Name| Description
| `xref:bsl/Pair_First-00.adoc[Pair_First<T1>]`
| Component‐private base holding and constructing a pair's `first` member.
| `xref:bsl/Pair_Second-05e.adoc[Pair_Second<T2>]`
| Component‐private base holding and constructing a pair's `second` member.
|===

== Type Aliases

[cols="1,4"]
|===
| Name| Description
| xref:bsl/pair-0b/first_type.adoc[`first_type`] 
| This `typedef` is an alias to the type of the `first` data member.
| xref:bsl/pair-0b/second_type.adoc[`second_type`] 
| This `typedef` is an alias to the type of the `second` data member.
|===


== Member Functions

[cols="1,4"]
|===
| Name| Description
| xref:bsl/pair-0b/2constructor-089.adoc[`pair`]         [.small]#[constructor]#
| Constructors
| xref:bsl/pair-0b/operator_assign-04.adoc[`operator=`] 
| Assignment operators
| xref:bsl/pair-0b/swap.adoc[`swap`] 
| Swap the value of this pair with the value of the specified `other` pair by applying `swap` to each of the `first` and `second` pair fields.
| xref:bsl/pair-0b/2conversion-057.adoc[`operator std::tuple<PARAM_1&, PARAM_2&>`] 
| Return an `std::tuple` object, holding references that provide modifiable access to the members of this object.
| xref:bsl/pair-0b/2conversion-0e.adoc[`operator std::tuple<PARAM_1&, decltype(std::ignore)&>`] 
| Convert to an `std::tuple` whose second element is `std::ignore`.
| xref:bsl/pair-0b/2conversion-054.adoc[`operator std::tuple<decltype(std::ignore)&, PARAM_2&>`] 
| Convert to an `std::tuple` whose first element is `std::ignore`.
| xref:bsl/pair-0b/2conversion-0c.adoc[`operator std::tuple<decltype(std::ignore)&, decltype(std::ignore)&>`] 
| Convert to an `std::tuple` whose elements are both `std::ignore`.
|===


== Using Declarations

[cols="1,4"]
|===
| Name| Description
| xref:bsl/pair-0b/first.adoc[`first`] 
| Bring the `first` data member from `FirstBase` into scope.
| xref:bsl/pair-0b/second.adoc[`second`] 
| Bring the `second` data member from `SecondBase` into scope.
|===


== Protected Data Members

[cols="1,4"]
|===
| Name| Description
| xref:bsl/Pair_First-00/first.adoc[`first`] 
| First element of the pair.
| xref:bsl/Pair_Second-05e/second.adoc[`second`] 
| Second element of the pair.
|===


== Deduction Guides

[cols="1,4"]
|===
| Name| Description
| xref:bsl/pair-04.adoc[`pair<T1, T2>`] 
| Deduce the specified types `T1` and `T2` from the corresponding template parameters of the `std::pair` supplied to the constructor of `pair`.
| xref:bsl/pair-09.adoc[`pair<T1, T2>`] 
| Deduce the specified types `T1` and `T2` from the corresponding types supplied to the constructor of `pair`.
| xref:bsl/pair-03.adoc[`pair<T1, T2>`] 
| Deduce `T1` and `T2` from the `bsl::pair` and allocator passed to `pair`.
| xref:bsl/pair-05.adoc[`pair<T1, T2>`] 
| Deduce `T1` and `T2` from the `std::pair` and allocator passed to `pair`.
| xref:bsl/pair-0f.adoc[`pair<T1, T2>`] 
| Deduce `T1` and `T2` from the values and allocator passed to `pair`.
|===


== Non-Member Functions

[cols="1,4"]
|===
| Name| Description
| xref:bsl/get-00c.adoc[`get`]
| Return a modifiable reference to the `INDEX`th element of `p`.
| xref:bsl/get-034.adoc[`get`]
| Return a non‐modifiable rvalue reference to the element of `p` of `TYPE`.
| xref:bsl/get-052.adoc[`get`]
| Return a non‐modifiable reference to the `INDEX`th element of `p`.
| xref:bsl/get-08.adoc[`get`]
| Return a non‐modifiable reference to the element of `p` having type `TYPE`.
| xref:bsl/get-0b0.adoc[`get`]
| Return a modifiable reference to the element of `p` having type `TYPE`.
| xref:bsl/get-0b3.adoc[`get`]
| Return a modifiable rvalue reference to the `INDEX`th element of `p`.
| xref:bsl/get-0e8.adoc[`get`]
| Return a modifiable rvalue reference to the element of `p` of type `TYPE`.
| xref:bsl/operator_3way-05.adoc[`operator<=>`]
| Compare the specified `lhs` and `rhs` pairs lexicographically.
| xref:bsl/operator_eq-0cf.adoc[`operator==`]
| Return `true` if the specified `lhs` and `rhs` pairs have the same value.
| xref:bsl/swap-0ce.adoc[`swap`]
| Swap the values of the specified `a` and `b` pairs.
|===


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