1+++
2title = "`const value_type &assume_value() const & noexcept`"
3description = "Narrow contract const lvalue reference observer of any value present. Constexpr propagating, never throws."
4categories = ["observers"]
5weight = 610
6+++
7
8Narrow contract const lvalue reference observer of any value present. `NoValuePolicy::narrow_value_check()` is first invoked, then the reference to the value is returned. The convention is that hard undefined behaviour occurs if no value is actually present, however `NoValuePolicy::narrow_value_check()` can do something to avoid that.
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10Note that if `value_type` is `void`, only a `const` overload returning `void` is present.
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12*Requires*: Always available.
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14*Complexity*: Depends on `NoValuePolicy::narrow_value_check()`.
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16*Guarantees*: An exception is never thrown.
17