# std::tuple<Types...>::operator=

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< cpp‎ | utility‎ | tuple

tuple& operator=( const tuple& other ); | (1) | (since C++11) (constexpr since C++20)
constexpr const tuple& operator=( const tuple& other ) const; | (2) | (since C++23)
tuple& operator=( tuple&& other ) noexcept(/* see below */); | (3) | (since C++11) (constexpr since C++20)
constexpr const tuple& operator=( tuple&& other ) const; | (4) | (since C++23)
template< class... UTypes > tuple& operator=( const tuple<UTypes...>& other ); | (5) | (since C++11) (constexpr since C++20)
template< class... UTypes > constexpr const tuple& operator=( const tuple<UTypes...>& other ) const; | (6) | (since C++23)
template< class... UTypes > tuple& operator=( tuple<UTypes...>&& other ); | (7) | (since C++11) (constexpr since C++20)
template< class... UTypes > constexpr const tuple& operator=( tuple<UTypes...>&& other ) const; | (8) | (since C++23)
template< class E1, class E2 > tuple& operator=( const std::pair <E1, E2>& p ); | (9) | (since C++11) (constexpr since C++20)
template< class E1, class E2 > constexpr const tuple& operator=( const std::pair <E1, E2>& p ) const; | (10) | (since C++23)
template< class E1, class E2 > tuple& operator=( std::pair <E1, E2>&& p ); | (11) | (since C++11) (constexpr since C++20)
template< class E1, class E2 > constexpr const tuple& operator=( std::pair <E1, E2>&& p ) const; | (12) | (since C++23)
template< tuple-like UTuple > constexpr tuple& operator=( UTuple&& u ); | (13) | (since C++23)
template< tuple-like UTuple > constexpr const tuple& operator=( UTuple&& u ) const; | (14) | (since C++23)

Replaces the contents of the tuple with the contents of another tuple-like object.

In the descriptions that follow, let

- i be in the range `[`​0​`,` sizeof...(Types)`)` in order,
- `Ti` be the `i`th type in the class template parameter pack `Types`, and
- `Ui` be the `i`th type in a function template parameter pack named `UTypes`,

where indexing is zero-based.

1) Copy assignment operator. Assigns each element of other to the corresponding element of *this.
This overload is defined as deleted unless std::is_copy_assignable <Ti>::value is true for all `Ti`.
2) Copy assignment operator for const-qualified operand. Assigns each element of other to the corresponding element of *this.
This overload participates in overload resolution only if std::is_copy_assignable_v <const Ti> is true for all `Ti`.
3) Move assignment operator. For all i, assigns std::forward <Ti>(std::get<i>(other)) to std::get<i>(*this).
This overload participates in overload resolution only if std::is_move_assignable <Ti>::value is true for all `Ti`.
4) Move assignment operator for const-qualified operand. For all i, assigns std::forward <Ti>(std::get<i>(other)) to std::get<i>(*this).
This overload participates in overload resolution only if std::is_assignable_v <const Ti&, Ti> is true for all `Ti`.
5) For all i, assigns std::get<i>(other) to std::get<i>(*this).
This overload participates in overload resolution only if sizeof...(Types) == sizeof...(UTypes), and std::is_assignable <Ti&, const Ui&>::value is true for all corresponding pairs of types `Ti` and `Ui`.
6) For all i, assigns std::get<i>(other) to std::get<i>(*this).
This overload participates in overload resolution only if sizeof...(Types) == sizeof...(UTypes), and std::is_assignable_v <const Ti&, const Ui&> is true for all corresponding pairs of types `Ti` and `Ui`.
7) For all i, assigns std::forward <Ui>(std::get<i>(other)) to std::get<i>(*this).
This overload participates in overload resolution only if sizeof...(Types) == sizeof...(UTypes), and std::is_assignable <Ti&, Ui>::value is true for all corresponding pairs of types `Ti`and `Ui`.
8) For all i, assigns std::forward <Ui>(std::get<i>(other)) to std::get<i>(*this).
This overload participates in overload resolution only if sizeof...(Types) == sizeof...(UTypes), and std::is_assignable_v <const Ti&, Ui> is true for all corresponding pairs of types `Ti` and `Ui`.
9) Assigns p.first to the first element of *this and p.second to the second element of *this.
This overload participates in overload resolution only if

- sizeof...(Types) == 2,
- std::is_assignable <T0&, const E1&>::value is true, and
- std::is_assignable <T1&, const E2&>::value is true.

10) Assigns p.first to the first element and p.second to the second element.
This overload participates in overload resolution only if

- sizeof...(Types) == 2,
- std::is_assignable_v <const T0&, const E1&> is true, and
- std::is_assignable_v <const T1&, const E2&> is true.

11) Assigns std::forward <E1>(p.first) to the first element of *this and std::forward <E2>(p.second) to the second element of *this.
This overload participates in overload resolution only if

- sizeof...(Types) == 2,
- std::is_assignable_v <T0&, E1> is true, and
- std::is_assignable_v <T1&, E2> is true.

12) Assigns std::forward <E1>(p.first) to the first element and std::forward <E2>(p.second) to the second element.
This overload participates in overload resolution only if

- sizeof...(Types) == 2,
- std::is_assignable_v <const T0&, E1> is true, and
- std::is_assignable_v <const T1&, E2> is true.

13) For all i, assigns std::get<i>( std::forward <UTuple>(u)) to std::get<i>(*this).
This overload participates in overload resolution only if

- std::same_as < std::remove_cvref_t <UTuple>, std::tuple > is false,
- std::remove_cvref_t <UTuple> is not a specialization of std::ranges::subrange,
- sizeof...(Types) equals std::tuple_size_v < std::remove_cvref_t <UTuple>>, and
- std::is_assignable_v <Ti&, decltype(std::get<i>( std::forward <UTuple>(u)))> is true for all i.

14) For all i, assigns std::get<i>( std::forward <UTuple>(u)) to std::get<i>(*this).
This overload participates in overload resolution only if

- std::same_as < std::remove_cvref_t <UTuple>, std::tuple > is false,
- std::remove_cvref_t <UTuple> is not a specialization of std::ranges::subrange,
- sizeof...(Types) equals std::tuple_size_v < std::remove_cvref_t <UTuple>>, and
- std::is_assignable_v <const Ti&, decltype(std::get<i>( std::forward <UTuple>(u)))> is true for all i.

### Parameters

other | - | tuple to replace the contents of this tuple
p | - | pair to replace the contents of this 2-tuple
u | - | `tuple-like` object to replace the contents of this tuple

### Return value

*this

### Exceptions

1,2) May throw implementation-defined exceptionsif the assignment of one of the types in `Types` throws an exception.
3)
`noexcept` specification:
noexcept(

std::is_nothrow_move_assignable <T0>::value &&
std::is_nothrow_move_assignable <T1>::value &&
std::is_nothrow_move_assignable <T2>::value &&
...

)
4-14) May throw implementation-defined exceptionsif the assignment of one of the types in `Types` throws an exception.

### Example

Run this code

    #include <iostream>
    #include <string>
    #include <string_view>
    #include <tuple>
    #include <utility>
    #include <vector>

    // helper function to print std::vector<int>
    std::ostream& operator<<(std::ostream& os, std::vector<int> const& v)
    {
        os << '{';
        for (std::size_t t = 0; t != v.size(); ++t)
            os << v[t] << (t + 1 < v.size() ? ", " : "");
        return os << '}';
    }

    // helpers to print a tuple of any size
    template<class... Args>
    void print_tuple(std::string_view name, const std::tuple<Args...>& t)
    {
        std::cout << name << " = {";
        std::apply([&](auto&& arg, auto&&... args)
        {
            std::cout << arg;
            ((std::cout << ", " << args), ...);
        }, t);
        std::cout << '}';
    }

    template<class Tuple1, class Tuple2>
    void print_tuples(std::string_view name1, const Tuple1& t1,
                      std::string_view name2, const Tuple2& t2)
    {
        print_tuple(name1, t1);
        std::cout << ", ";
        print_tuple(name2, std::tuple(t2));
        std::cout << "\n\n";
    }

    int main()
    {
        // Tuple to tuple examples //
        std::tuple<int, std::string, std::vector<int>>
            t1{1, "alpha", {1, 2, 3}},
            t2{2, "beta", {4, 5}};
        print_tuples("1) t1", t1, "t2", t2);

        // Normal copy assignment
        // operator=( const tuple& other );
        t1 = t2;
        print_tuples("2) t1 = t2;\n   t1", t1, "t2", t2);

        // Normal move assignment
        // operator=( tuple&& other );
        t1 = std::move(t2);
        print_tuples("3) t1 = std::move(t2);\n   t1", t1, "t2", t2);

        // Converting copy assignment
        // operator=( const tuple<UTypes...>& other );
        std::tuple<short, const char*, std::vector<int>> t3{3, "gamma", {6, 7, 8}};
        t1 = t3;
        print_tuples("4) t1 = t3;\n   t1", t1, "t3", t3);

        // Converting move assignment
        // operator=( tuple<UTypes...>&& other );
        t1 = std::move(t3);
        print_tuples("5) t1 = std::move(t3);\n   t1", t1, "t3", t3);

        // Pair to tuple examples //
        std::tuple<std::string, std::vector<int>> t4{"delta", {10, 11, 12}};
        std::pair<const char*, std::vector<int>> p1{"epsilon", {14, 15, 16}};
        print_tuples("6) t4", t4, "p1", p1);

        // Converting copy assignment from std::pair
        // operator=( const std::pair<U1, U2>& p );
        t4 = p1;
        print_tuples("7) t4 = p1;\n   t4", t4, "p1", p1);

        // Converting move assignment from std::pair
        // operator=( std::pair<U1, U2>&& p );
        t4 = std::move(p1);
        print_tuples("8) t4 = std::move(p1);\n   t4", t4, "p1", p1);
    }

Possible output:

    1) t1 = {1, alpha, {1, 2, 3}}, t2 = {2, beta, {4, 5}}

    2) t1 = t2;
       t1 = {2, beta, {4, 5}}, t2 = {2, beta, {4, 5}}

    3) t1 = std::move(t2);
       t1 = {2, beta, {4, 5}}, t2 = {2, , {}}

    4) t1 = t3;
       t1 = {3, gamma, {6, 7, 8}}, t3 = {3, gamma, {6, 7, 8}}

    5) t1 = std::move(t3);
       t1 = {3, gamma, {6, 7, 8}}, t3 = {3, gamma, {}}

    6) t4 = {delta, {10, 11, 12}}, p1 = {epsilon, {14, 15, 16}}

    7) t4 = p1;
       t4 = {epsilon, {14, 15, 16}}, p1 = {epsilon, {14, 15, 16}}

    8) t4 = std::move(p1);
       t4 = {epsilon, {14, 15, 16}}, p1 = {epsilon, {}}

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 2729 | C++11 | operator= was unconstrained and might result in unnecessary undefined behavior | constrained

### See also

(constructor) | constructs a new `tuple` (public member function)
operator= | assigns the contents (public member function of `std::pair<T1,T2>`)

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