tmp/tmpo6syl047/{from.md → to.md}
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| 1 |
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#### Monadic operations <a id="optional.ref.monadic">[[optional.ref.monadic]]</a>
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``` cpp
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template<class F> constexpr auto and_then(F&& f) const;
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```
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Let `U` be `invoke_result_t<F, T&>`.
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*Mandates:* `remove_cvref_t<U>` is a specialization of `optional`.
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*Effects:* Equivalent to:
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``` cpp
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if (has_value()) {
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return invoke(std::forward<F>(f), *val);
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} else {
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return remove_cvref_t<U>();
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}
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```
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``` cpp
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template<class F>
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constexpr optional<remove_cv_t<invoke_result_t<F, T&>>> transform(F&& f) const;
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```
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Let `U` be `remove_cv_t<invoke_result_t<F, T&>>`.
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*Mandates:* The declaration
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``` cpp
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U u(invoke(std::forward<F>(f), *val));
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```
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is well-formed for some invented variable `u`.
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[*Note 1*: There is no requirement that `U` is
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movable [[dcl.init.general]]. — *end note*]
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*Returns:* If `*this` contains a value, an `optional<U>` object whose
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contained value is direct-non-list-initialized with
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`invoke(std::forward<F>(f), *`*`val`*`)`; otherwise, `optional<U>()`.
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``` cpp
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template<class F> constexpr optional or_else(F&& f) const;
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```
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*Constraints:* `F` models `invocable`.
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*Mandates:* `is_same_v<remove_cvref_t<invoke_result_t<F>>, optional>` is
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`true`.
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*Effects:* Equivalent to:
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``` cpp
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if (has_value()) {
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return *val;
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} else {
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return std::forward<F>(f)();
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}
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```
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