std::expected<T,E>::swap
Swaps the contents with those of other.
Value ofhas_value()
|
Value of other.has_value() | |
|---|---|---|
| true | false | |
| true | using std::swap ; swap( val , rhs.val);
|
see below |
| false | other.swap(*this); | using std::swap ; swap( unex , rhs.unex);
|
has_value() is true and other.has_value() is false, equivalent to:
// Case 1: the move constructions of unexpected values are non-throwing:
// "other.unex" will be restored if the construction of "other.val" fails
if constexpr (std::is_nothrow_move_constructible_v <E>)
{
E temp(std::move(other.unex));
std::destroy_at (std::addressof (other.unex));
try
{
std::construct_at (std::addressof (other.val), std::move(val)); // may throw
std::destroy_at (std::addressof (val));
std::construct_at (std::addressof (unex), std::move(temp));
}
catch(...)
{
std::construct_at (std::addressof (other.unex), std::move(temp));
throw;
}
}
// Case 2: the move constructions of expected values are non-throwing:
// "this->val" will be restored if the construction of "this->unex" fails
else
{
T temp(std::move(val));
std::destroy_at (std::addressof (val));
try
{
std::construct_at (std::addressof (unex), std::move(other.unex)); // may throw
std::destroy_at (std::addressof (other.unex));
std::construct_at (std::addressof (other.val), std::move(temp));
}
catch(...)
{
std::construct_at (std::addressof (val), std::move(temp));
throw;
}
}
has_val = false;
rhs.has_val = true;
Value ofhas_value()
|
Value of other.has_value() | |
|---|---|---|
| true | false | |
| true | using std::swap ; swap( val , rhs.val);
|
std::construct_at (std::addressof (unex), std::move(rhs.unex));std::destroy_at (std::addressof (rhs. unex));has_val = false;rhs. has_val = true;
|
| false | other.swap(*this); | using std::swap ; swap( unex , rhs.unex);
|
Contents
[edit] Parameters
expected object to exchange the contents with
[edit] Exceptions
std::is_nothrow_move_constructible_v <T> && std::is_nothrow_swappable_v <T> &&
std::is_nothrow_move_constructible_v <E> && std::is_nothrow_swappable_v <E>
std::is_nothrow_move_constructible_v <E> && std::is_nothrow_swappable_v <E>
[edit] Example
#include <expected> #include <iostream> #include <string_view> using Ex = std::expected <std::string, int>; void show(const Ex& ex1, const Ex& ex2, std::string_view term = "\n") { for (int i{}; i != 2; ++i) { std::cout << (i ? "ex2" : "ex1"); if (const Ex& ex = (i ? ex2 : ex1); ex.has_value()) std::cout << ".value() = " << *ex << " "; else std::cout << ".error() = " << ex.error() << " "; } std::cout << term; } int main() { Ex ex1("\N{CAT FACE}"); Ex ex2{"\N{GREEN HEART}"}; show(ex1, ex2, "after ex1.swap(ex2):\n"); ex1.swap(ex2); show(ex1, ex2, "\n\n"); ex2 = std::unexpected (13); show(ex1, ex2, "after ex1.swap(ex2):\n"); ex1.swap(ex2); show(ex1, ex2, "\n\n"); ex2 = std::unexpected (37); show(ex1, ex2, "after ex1.swap(ex2):\n"); ex1.swap(ex2); show(ex1, ex2); }
Output:
ex1.value() = 🐱 ex2.value() = 💚 after ex1.swap(ex2): ex1.value() = 💚 ex2.value() = 🐱 ex1.value() = 💚 ex2.error() = 13 after ex1.swap(ex2): ex1.error() = 13 ex2.value() = 💚 ex1.error() = 13 ex2.error() = 37 after ex1.swap(ex2): ex1.error() = 37 ex2.error() = 13