std::optional<T>::and_then
template< class F > constexpr auto and_then( F&& f ) &; |
(1) | (since C++23) |
template< class F > constexpr auto and_then( F&& f ) const&; |
(2) | (since C++23) |
template< class F > constexpr auto and_then( F&& f ) &&; |
(3) | (since C++23) |
template< class F > constexpr auto and_then( F&& f ) const&&; |
(4) | (since C++23) |
Returns the result of invocation of f on the contained value if it exists. Otherwise, returns an empty value of the return type.
The return type (see below) must be a specialization of std::optional. Otherwise, the program is ill-formed.
if (*this)
return std::invoke(std::forward<F>(f), this->value());
else
.
return std::remove_cvref_t<std::invoke_result_t<F, T&>>();.
if (*this)
return std::invoke(std::forward<F>(f), this->value());
else
.
return std::remove_cvref_t<std::invoke_result_t<F, const T&>>();.
if (*this)
return std::invoke(std::forward<F>(f), std::move(this->value()));
else
.
return std::remove_cvref_t<std::invoke_result_t<F, T>>();.
if (*this)
return std::invoke(std::forward<F>(f), std::move(this->value()));
else
.
return std::remove_cvref_t<std::invoke_result_t<F, const T>>();.
Parameters
| f | - | a suitable function or Callable object that returns an std::optional |
Return value
The result of f or an empty std::optional, as described above.
Notes
Some languages call this operation flatmap.
Feature testing macro: __cpp_lib_monadic_optional.
Example
#include <optional>
#include <vector>
#include <iostream>
#include <string>
#include <charconv>
#include <ranges>
std::optional<int> to_int(std::string_view sv)
{
int r{};
auto [ptr, ec] { std::from_chars(sv.data(), sv.data() + sv.size(), r) };
if (ec == std::errc())
return r;
else
return std::nullopt;
}
int main()
{
using namespace std::literals;
std::vector<std::optional<std::string>> v = {
"1234", "15 foo", "bar", "42", "5000000000", " 5"
};
for( auto&& x : v | std::views::transform([](auto&& o) { return
o.and_then(to_int) // flatmap from strings to ints
// (making empty optionals where it fails)
.transform([](int n) { return n+1; }) // map int to int + 1
.transform([](int n) { return std::to_string(n); }) // convert back to strings
.or_else([]{ return std::optional{"Null"s}; // replace all empty optionals
// that were left by and_then and
// ignored by transforms with "Null"
}); }) )
std::cout << *x << '\n';
}
Output:
1235 16 Null 43 Null Null
See also
| returns the contained value if available, another value otherwise (public member function) |
|
|
(C++23)
|
returns an optional containing the transformed contained value if it exists, or an empty optional otherwise (public member function) |
|
(C++23)
|
returns the optional itself if it contains a value, or the result of the given function otherwise (public member function) |
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