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

 

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Utilities library

Language support Type support (basic types, RTTI) Library feature-test macros (C++20) Program utilities Variadic functions initializer_list (C++11) is_constant_evaluated (C++20) is_within_lifetime (C++26) source_location (C++20) Coroutine support (C++20) Contract support (C++26) Three-way comparison three_way_comparablethree_way_comparable_with (C++20)(C++20) strong_ordering (C++20) weak_ordering (C++20) partial_ordering (C++20) common_comparison_category (C++20) compare_three_way_result (C++20) compare_three_way (C++20) strong_order (C++20) weak_order (C++20) partial_order (C++20) compare_strong_order_fallback (C++20) compare_weak_order_fallback (C++20) compare_partial_order_fallback (C++20)   is_eqis_ltis_lteq (C++20)(C++20)(C++20)   is_neqis_gtis_gteq (C++20)(C++20)(C++20)

General utilities Function objects Bit manipulation (C++20) bitset hash (C++11) Relational operators (deprecated in C++20) rel_ops::operator!=rel_ops::operator>    rel_ops::operator<=rel_ops::operator>= Integer comparison functions cmp_equalcmp_lesscmp_less_than (C++20)(C++20)(C++20)   cmp_not_equalcmp_greatercmp_greater_than (C++20)(C++20)(C++20) in_range (C++20) Swap and type operations swap ranges::swap (C++20) exchange (C++14) declval (C++11) to_underlying (C++23) forward (C++11) forward_like (C++23) move (C++11) move_if_noexcept (C++11) as_const (C++17) Common vocabulary types pair tuple (C++11) optional (C++17) any (C++17) variant (C++17) tuple_size (C++11) tuple_element (C++11) apply (C++17) make_from_tuple (C++17) expected (C++23)

 

std::tuple

Member functions

tuple::tuple

tuple::operator=

tuple::swap

Non-member functions

make_tuple

tie

forward_as_tuple

tuple_cat

operator==operator!=operator<operator<=operator>operator>=operator<=> (until C++20)(until C++20)(until C++20)(until C++20)(until C++20)(C++20)

swap(std::tuple)

get(std::tuple)

Helper concepts

tuple-like (C++23)

Helper classes

tuple_size<std::tuple>

tuple_element<std::tuple>

uses_allocator<std::tuple>

basic_common_reference<std::tuple> (C++23)

common_type<std::tuple> (C++23)

formatter<std::tuple> (C++23)

ignore

Deduction guides(C++17)

 

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)

See More

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 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>)