std::variant::valueless_by_exception

 

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constexpr bool valueless_by_exception() const;		(since C++17)

Returns false if and only if the variant holds a value.

Exceptions

noexcept specification:  

noexcept

  

Notes

A variant may become valueless in the following situations:

• (guaranteed) an exception is thrown during the move initialization of the contained value from the temporary in copy assignment
• (guaranteed) an exception is thrown during the move initialization of the contained value during move assignment
• (optionally) an exception is thrown when initializing the contained value during a type-changing assignment
• (optionally) an exception is thrown when initializing the contained value during a type-changing emplace

Since variant is never permitted to allocate dynamic memory, previous value cannot be retained in these situations.

This applies even to variants of non-class types:

struct S {
    operator int() { throw 42; }
};
variant<float, int> v{12.f}; // OK
v.emplace<1>(S()); // v may be valueless

A variant that is valueless by exception is treated as being in an invalid state: index returns variant_npos, get and visit throw bad_variant_access.