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std::optional<T>::and_then

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< cpp‎ | utility‎ | optional
 
 
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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.

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>>();

Contents

[edit] Parameters

f - a suitable function or Callable object that returns an std::optional

[edit] Return value

The result of f or an empty std::optional, as described above.

[edit] Notes

Some languages call this operation flatmap.

Feature-test macro Value Std
__cpp_lib_optional 202110L (C++23)

[edit] 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

[edit] See also

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