std::tuple::operator=(3)

NAME
std::tuple::operator= - std::tuple::operator=

Synopsis
   tuple& operator=( const tuple& other ); (1) (since C++11)
                        (constexpr since C++20)
   constexpr const tuple& operator=( const tuple& other ) (2) (since C++23)
   const;
   tuple& operator=( tuple&& other ) noexcept(/* see below (3) (since C++11)
   */); (constexpr since C++20)
   constexpr const tuple& operator=( tuple&& other ) (4) (since C++23)
   const;
   template< class... UTypes > (5) (since C++11)
   tuple& operator=( const tuple<UTypes...>& other ); (constexpr since C++20)
   template< class... UTypes >
   constexpr const tuple& operator=( const (6) (since C++23)
   tuple<UTypes...>& other ) const;
   template< class... UTypes > (7) (since C++11)
   tuple& operator=( tuple<UTypes...>&& other ); (constexpr since C++20)
   template< class... UTypes >
   constexpr const tuple& operator=( tuple<UTypes...>&& (8) (since C++23)
   other ) const;
   template< class E1, class E2 > (9) (since C++11)
   tuple& operator=( const std::pair<E1, E2>& p ); (constexpr since C++20)
   template< class E1, class E2 >
   constexpr const tuple& operator=( const std::pair<E1, (10) (since C++23)
   E2>& p ) const;
   template< class E1, class E2 > (11) (since C++11)
   tuple& operator=( std::pair<E1, E2>&& p ); (constexpr since C++20)
   template< class E1, class E2 >
   constexpr const tuple& operator=( std::pair<E1, E2>&& p (12) (since C++23)
   ) const;
   template< tuple-like UTuple > (13) (since C++23)
   constexpr tuple& operator=( UTuple&& u );
   template< tuple-like UTuple > (14) (since C++23)
   constexpr const tuple& operator=( UTuple&& u ) const;
   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 ith type in the class template parameter pack Types, and
     * Ui be the ith 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 Tiand 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 constrained
                       result in unnecessary undefined behavior

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

Category:
     * conditionally noexcept