NAME¶

std::optional::and_then - std::optional::and_then

Synopsis¶

template< class F > (1) (since C++23)
constexpr auto and_then( F&& f ) &;
template< class F > (2) (since C++23)
constexpr auto and_then( F&& f ) const&;
template< class F > (3) (since C++23)
constexpr auto and_then( F&& f ) &&;
template< class F > (4) (since C++23)
constexpr auto and_then( F&& f ) const&&;

If *this contains a value, invokes f with the contained value as an argument, and
returns the result of that invocation; otherwise, returns an empty std::optional.

The return type (see below) must be a specialization of std::optional (unlike
transform()). Otherwise, the program is ill-formed.

1) Equivalent to

if (*this)
return std::invoke(std::forward<F>(f), **this);
else
return std::remove_cvref_t<std::invoke_result_t<F, T&>>{};

2) Equivalent to

if (*this)
return std::invoke(std::forward<F>(f), **this);
else
return std::remove_cvref_t<std::invoke_result_t<F, const T&>>{};

3) Equivalent to

if (*this)
return std::invoke(std::forward<F>(f), std::move(**this));
else
return std::remove_cvref_t<std::invoke_result_t<F, T>>{};

4) Equivalent to

if (*this)
return std::invoke(std::forward<F>(f), std::move(**this));
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-test macro Value Std Feature
__cpp_lib_optional 202110L (C++23) Monadic operations in std::optional

Example¶

// Run this code

#include <charconv>
#include <iomanip>
#include <iostream>
#include <optional>
#include <ranges>
#include <string>
#include <string_view>
#include <vector>

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;

const std::vector<std::optional<std::string>> v
{
"1234", "15 foo", "bar", "42", "5000000000", " 5", std::nullopt, "-43"
};

for (auto&& x : v | std::views::transform(
[](auto&& o)
{
// debug print the content of input optional<string>
std::cout << std::left << std::setw(13)
<< std::quoted(o.value_or("nullopt")) << " -> ";

return o
// if optional is nullopt convert it to optional with "" string
.or_else([]{ return std::optional{""s}; })
// flatmap from strings to ints (making empty optionals where it fails)
.and_then(to_int)
// map int to int + 1
.transform([](int n) { return n + 1; })
// convert back to strings
.transform([](int n) { return std::to_string(n); })
// replace all empty optionals that were left by
// and_then and ignored by transforms with "NaN"
.value_or("NaN"s);
}))
std::cout << x << '\n';
}

Output:¶

"1234" -> 1235
"15 foo" -> 16
"bar" -> NaN
"42" -> 43
"5000000000" -> NaN
" 5" -> NaN
"nullopt" -> NaN
"-43" -> -42

See also¶

value_or returns the contained value if available, another value otherwise
(public member function)
transform returns an optional containing the transformed contained value if it
(C++23) exists, or an empty optional otherwise
(public member function)
or_else returns the optional itself if it contains a value, or the result of the
(C++23) given function otherwise
(public member function)