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Showing content from https://en.cppreference.com/w/cpp/language/../algorithm/../language/../utility/tuple/operator=.html below:

std::tuple<Types...>::operator= - cppreference.com

tuple& operator=( const tuple& other );

constexpr const tuple& operator=( const tuple& other ) const;

tuple& operator=( tuple&& other ) noexcept(/* see below */);

constexpr const tuple& operator=( tuple&& other ) const;

template< class... UTypes >
tuple& operator=( const tuple<UTypes...>& other );

template< class... UTypes >
constexpr const tuple& operator=( const tuple<UTypes...>& other ) const;

template< class... UTypes >
tuple& operator=( tuple<UTypes...>&& other );

template< class... UTypes >
constexpr const tuple& operator=( tuple<UTypes...>&& other ) const;

template< tuple-like UTuple >
constexpr tuple& operator=( UTuple&& u );

template< tuple-like UTuple >
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.

2) Copy assignment operator for const-qualified operand. Assigns each element of other to the corresponding element of *this.

Move assignment operator. For all

, assigns

to

.

Move assignment operator for const-qualified operand. For all

, assigns

to

.

This overload participates in overload resolution only if

is

for all

.

5) For all i, assigns std::get<i>(other) to std::get<i>(*this).

This overload participates in overload resolution only if

, and

is

for all corresponding pairs of types

and

.

6) For all i, assigns std::get<i>(other) to std::get<i>(*this).

This overload participates in overload resolution only if

, and

is

for all corresponding pairs of types

and

.

For all

, assigns

to

.

This overload participates in overload resolution only if

, and

is

for all corresponding pairs of types

and

.

For all

, assigns

to

.

This overload participates in overload resolution only if

, and

is

for all corresponding pairs of types

and

.

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.

Assigns

to the first element of

and

to the second element of

.

For all

, assigns

to

.

For all

, assigns

to

.

*this

1,2) May throw implementation-defined exceptionsif the assignment of one of the types in Types throws an exception.

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

#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, {}}

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

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