std::tuple::operator=(3)

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

Synopsis
                        (since
   tuple& operator=( const C++11)
   tuple& other ); (until
                        C++20)
   constexpr tuple& operator=( (since
   const tuple& other ); C++20)
   constexpr const tuple& (since
   operator=( const tuple& (2) C++23)
   other ) const;
   tuple& operator=( tuple&& (since
   other ) noexcept(/* see C++11)
   below */); (until
                        C++20)
   constexpr tuple& operator=( (since
   tuple&& other ) noexcept(/* C++20)
   see below */);
   constexpr const tuple& (since
   operator=( tuple&& other ) (4) C++23)
   const;
   template< class... UTypes > (since
   tuple& operator=( const C++11)
   tuple<UTypes...>& other ); (until
                        C++20)
   template< class... UTypes >
   constexpr tuple& operator=( (since
   const tuple<UTypes...>& C++20)
   other );
   template< class... UTypes >
   constexpr const tuple& (since
   operator=( const (6) C++23)
   tuple<UTypes...>& other )
   const;
   template< class... UTypes > (since
   tuple& operator=( C++11)
   tuple<UTypes...>&& other ); (until
                        (1) C++20)
   template< class... UTypes > (since
   constexpr tuple& operator=( C++20)
   tuple<UTypes...>&& other );
   template< class... UTypes >
   constexpr const tuple& (3) (since
   operator=( (8) C++23)
   tuple<UTypes...>&& other )
   const;
   template< class U1, class U2 (since
   > (5) C++11)
   tuple& operator=( const (until
   std::pair<U1, U2>& p ); C++20)
   template< class U1, class U2
   > (since
   constexpr tuple& operator=( C++20)
   const std::pair<U1, U2>& p (7)
   );
   template< class U1, class U2
   >
   constexpr const tuple& (10) (since
   operator=( const C++23)
   std::pair<U1, U2>& p ) (9)
   const;
   template< class U1, class U2 (since
   > C++11)
   tuple& operator=( (until
   std::pair<U1, U2>&& p ); C++20)
   template< class U1, class U2
   > (since
   constexpr tuple& operator=( (11) C++20)
   std::pair<U1, U2>&& p );
   template< class U1, class U2
   > (since
   constexpr const tuple& (12) C++23)
   operator=( std::pair<U1,
   U2>&& p ) const;
   Replaces the contents of the tuple with the contents of another tuple or a pair.
   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<T_i>::value
       is true for all T_i in Types.
   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 T_i> is true for all T_i in Types.
   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<T_i>::value is true for all T_i in Types.
   4) Move assignment operator 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 T_i&, T_i> is true for all T_i in Types.
   5) For all i, assigns std::get<i>(other) to std::get<i>(*this).
     * This overload participates in overload resolution only if sizeof...(UTypes) ==
       sizeof...(Types) and std::is_assignable<T_i&, const U_i&>::value is true for all
       corresponding pairs of types T_i in Types and U_i in UTypes.
   6) For all i, assigns std::get<i>(other) to std::get<i>(*this).
     * This overload participates in overload resolution only if sizeof...(UTypes) ==
       sizeof...(Types) and std::is_assignable_v<const T_i&, const U_i&> is true for
       all corresponding pairs of types T_i in Types and U_i in UTypes.
   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...(UTypes) ==
       sizeof...(Types) and std::is_assignable<T_i&, U_i>::value is true for all
       corresponding pairs of types T_i in Types and U_i in UTypes.
   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...(UTypes) ==
       sizeof...(Types) and std::is_assignable_v<const T_i&, U_i> is true for all
       corresponding pairs of types T_i in Types and U_i in UTypes.
   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<T_0&, const U1&>::value and std::is_assignable<T_1&, const
       U2&>::value are both true, where T_0 and T_1 are the two types constituting

Types.
   10) 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_v<const T_0&, const U1&> and std::is_assignable_v<const T_1&,
       const U2&> are both true, where T_0 and T_1 are the two types constituting

Types.
   11) Assigns std::forward<U1>(p.first) to the first element of *this and
   std::forward<U2>(p.second) to the second element of *this.
     * This overload participates in overload resolution only if
       std::is_assignable<T_0&, U1>::value and std::is_assignable<T_1&, U2>::value are
       both true, where T_0 and T_1 are the two types constituting Types.
   12) Assigns std::forward<U1>(p.first) to the first element of *this and
   std::forward<U2>(p.second) to the second element of *this.
     * This overload participates in overload resolution only if
       std::is_assignable<const T_0&, U1> and std::is_assignable_v<const T_1&, U2> are
       both true, where T_0 and T_1 are the two types constituting Types.

Parameters
   other - tuple to replace the contents of this tuple
   p - pair to replace the contents of this 2-tuple

Return value
   *this

Exceptions
   1,2) May throw implementation-defined exceptions.
   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 &&
   ...
   )
   4-12) May throw implementation-defined exceptions.

Example
// Run this code
 #include <iostream>
 #include <string>
 #include <tuple>
 #include <utility>
 #include <vector>
 // helper function to print std::vector
 template<class Os, class T>
 Os& operator<< (Os& os, std::vector<T> 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 Os, class... Args>
 Os& operator<< (Os& os, const std::tuple<Args...>& t) {
     os << "{ ";
     std::apply([&](auto&& arg, auto&&... args) {
         os << arg;
         ((os << ", " << args), ...);
     }, t);
     return os << " }";
 }
 struct line { int len{60}; };
 template<class Os>
 Os& operator<< (Os& os, line l) {
     while (l.len-- > 0)
         std::cout << "─";
     return os << '\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} };
     // Normal copy assignment
     // operator=( const tuple& other );
     std::cout << "t1 = " << t1 << ", t2 = " << t2 << '\n';
     t1 = t2;
     std::cout << "t1 = t2;\n" "t1 = " << t1 << ", t2 = " << t2 << '\n' << line{};
     // Normal move assignment
     // operator=( tuple&& other );
     t1 = std::move(t2);
     std::cout << "t1 = std::move(t2);\n" "t1 = " << t1 << ", t2 = " << t2 << '\n' << line{};
     // Converting copy assignment
     // operator=( const tuple<UTypes...>& other );
     std::tuple<short, const char*, std::vector<int>> t3{3, "gamma", {6,7,8} };
     t1 = t3;
     std::cout << "t1 = t3; \n" "t1 = " << t1 << ", t3 = " << t3 << '\n' << line{};
     // Converting move assignment
     // operator=( tuple<UTypes...>&& other );
     t1 = std::move(t3);
     std::cout << "t1 = std::move(t3);\n" "t1 = " << t1 << ", t3 = " << t3 << '\n' << line{};
     // 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} };
     // Converting copy assignment from std::pair
     // operator=( const std::pair<U1,U2>& p );
     std::cout << "t4 = " << t4 << ", "
               << "p1 = { " << p1.first << ", " << p1.second << " };\n";
     t4 = p1;
     std::cout << "t4 = p1;\n" "t4 = " << t4
               << ", p1 = { " << p1.first << ", " << p1.second << " };\n" << line{};
     // Converting move assignment from std::pair
     // operator=( std::pair<U1,U2>&& p );
     t4 = std::move(p1);
     std::cout << "t4 = std::move(p1);\n" "t4 = " << t4
               << ", p1 = { " << p1.first << ", " << p1.second << " };\n" << line{};
 #ifdef __cpp_lib_ranges_zip
     // Const tuple-of-proxies assignment example
     std::vector<bool> v({false, true});
     const std::tuple<std::vector<bool>::reference> t0_const{v[0]}, t1_const{v[1]};
     t0_const = t1_const;
     std::cout << std::boolalpha << "t0_const = t1_const;\n" "t0_const = " << t0_const
               << ", t1_const = " << t1_const << '\n';
 #endif
 }

Possible output:
 t1 = { 1, alpha, {1,2,3} }, t2 = { 2, beta, {4,5} }
 t1 = t2;
 t1 = { 2, beta, {4,5} }, t2 = { 2, beta, {4,5} }
 ────────────────────────────────────────────────────────────
 t1 = std::move(t2);
 t1 = { 2, beta, {4,5} }, t2 = { 2, , {} }
 ────────────────────────────────────────────────────────────
 t1 = t3;
 t1 = { 3, gamma, {6,7,8} }, t3 = { 3, gamma, {6,7,8} }
 ────────────────────────────────────────────────────────────
 t1 = std::move(t3);
 t1 = { 3, gamma, {6,7,8} }, t3 = { 3, gamma, {} }
 ────────────────────────────────────────────────────────────
 t4 = { delta, {10,11,12} }, p1 = { epsilon, {14,15,16} };
 t4 = p1;
 t4 = { epsilon, {14,15,16} }, p1 = { epsilon, {14,15,16} };
 ────────────────────────────────────────────────────────────
 t4 = std::move(p1);
 t4 = { epsilon, {14,15,16} }, p1 = { epsilon, {} };
 ────────────────────────────────────────────────────────────
 t0_const = t1_const;
 t0_const = { true }, t1_const = { true }
  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 tuple::operator= was unconstrained and might constrained
                       result in unnecessary undefined behavior

See also
   constructor constructs a new tuple
   (C++11) (public member function)
   operator= assigns the contents
                 (public member function of std::pair<T1,T2>)