std::variant::valueless_by_exception(3)

NAME
std::variant::valueless_by_exception - std::variant::valueless_by_exception

Synopsis
   constexpr bool valueless_by_exception() const noexcept; (since C++17)
   Returns false if and only if the variant holds a value.

Notes
   A variant may become valueless in the following situations:
     * (guaranteed) an exception is thrown during the initialization of the contained
       value during move assignment
     * (optionally) an exception is thrown during the initialization of the contained
       value during copy 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. The situations marked "optionally" can be worked
   around by implementations that first construct the new value on the stack and then
   move it into the variant (provided non-throwing move).
   This applies even to variants of non-class types:
 struct S {
     operator int() { throw 42; }
 };
 std::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.

Example
// Run this code
 #include <cassert>
 #include <iostream>
 #include <stdexcept>
 #include <string>
 #include <variant>
 struct Demo {
     Demo(int) {}
     Demo(const Demo&) { throw std::domain_error("copy ctor"); }
     Demo& operator= (const Demo&) = default;
 };
 int main()
 {
     std::variant<std::string, Demo> var{"str"};
     assert(var.index() == 0);
     assert(std::get<0>(var) == "str");
     assert(var.valueless_by_exception() == false);
     try {
         var = Demo{555};
     } catch (const std::domain_error& ex) {
         std::cout << "1) Exception: " << ex.what() << '\n';
     }
     assert(var.index() == std::variant_npos);
     assert(var.valueless_by_exception() == true);
     // Now the var is "valueless" which is an invalid state caused
     // by an exception raised in the process of type-changing assignment.
     try {
         std::get<1>(var);
     } catch (const std::bad_variant_access& ex) {
         std::cout << "2) Exception: " << ex.what() << '\n';
     }
     var = "str2";
     assert(var.index() == 0);
     assert(std::get<0>(var) == "str2");
     assert(var.valueless_by_exception() == false);
 }

Possible output:
 1) Exception: copy ctor
 2) Exception: std::get: variant is valueless

See also
   get(std::variant) reads the value of the variant given the index or the type (if
   (C++17) the type is unique), throws on error
                      (function template)
                      returns the zero-based index of the alternative held by the
   index variant
                      (public member function)
   bad_variant_access exception thrown on invalid accesses to the value of a variant
   (C++17) (class)