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