std::variant::valueless_by_exception
From cppreference.com
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.