W3cubDocs

/C++

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)

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->value());
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->value());
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->value()));
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->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-test macro	Value	Std	Comment
`__cpp_lib_optional`	202110L	(C++23)	Monadic operations in `std::optional`

Example

#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

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

std::optional<T>::and_then

Parameters

Return value

Notes

Example

See also