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std::any_cast(3) C++ Standard Libary std::any_cast(3) NAME std::any_cast - std::any_cast Synopsis Defined in header <any> template< class T > (1) (since C++17) T any_cast( const any& operand ); template< class T > (2) (since C++17) T any_cast( any& operand ); template< class T > (3) (since C++17) T any_cast( any&& operand ); template< class T > (4) (since C++17) const T* any_cast( const any* operand ) noexcept; template< class T > (5) (since C++17) T* any_cast( any* operand ) noexcept; Performs type-safe access to the contained object. Let U be std::remove_cv_t<std::remove_reference_t<T>>. 1) The program is ill-formed if std::is_constructible_v<T, const U&> is false. 2) The program is ill-formed if std::is_constructible_v<T, U&> is false. 3) The program is ill-formed if std::is_constructible_v<T, U> is false. 4,5) The program is ill-formed if std::is_void_v<T> is true. Parameters operand - target any object Return value 1,2) Returns static_cast<T>(*std::any_cast<U>(&operand)). 3) Returns static_cast<T>(std::move(*std::any_cast<U>(&operand))). 4,5) If operand is not a null pointer, and the typeid of the re- quested T matches that of the contents of operand, a pointer to the value contained by operand, otherwise a null pointer. Exceptions 1-3) Throws std::bad_any_cast if the typeid of the requested T does not match that of the contents of operand. Example // Run this code #include <any> #include <iostream> #include <string> #include <type_traits> #include <utility> int main() { // Simple example auto a1 = std::any(12); std::cout << "1) a1 is int: " << std::any_cast<int>(a1) << '\n'; try { auto s = std::any_cast<std::string>(a1); // throws } catch (const std::bad_any_cast& e) { std::cout << "2) " << e.what() << '\n'; } // Pointer example if (int* i = std::any_cast<int>(&a1)) std::cout << "3) a1 is int: " << *i << '\n'; else if (std::string* s = std::any_cast<std::string>(&a1)) std::cout << "3) a1 is std::string: " << *s << '\n'; else std::cout << "3) a1 is another type or unset\n"; // Advanced example a1 = std::string("hello"); auto& ra = std::any_cast<std::string&>(a1); //< reference ra[1] = 'o'; std::cout << "4) a1 is string: " << std::any_cast<std::string const&>(a1) << '\n'; //< const reference auto s1 = std::any_cast<std::string&&>(std::move(a1)); //< rvalue reference // Note: s1 is a move-constructed std::string: static_assert(std::is_same_v<decltype(s1), std::string>); // Note: the std::string in a1 is left in valid but unspecified state std::cout << "5) a1.size(): " << std::any_cast<std::string>(&a1)->size() //< pointer << '\n' << "6) s1: " << s1 << '\n'; } Possible output: 1) a1 is int: 12 2) bad any_cast 3) a1 is int: 12 4) a1 is string: hollo 5) a1.size(): 0 6) s1: hollo Defect reports The following behavior-changing defect reports were applied retroac- tively to previously published C++ standards. DR Applied to Behavior as published Cor- rect behavior LWG 3305 C++17 the behavior of overloads (4,5) was the pro- gram ill-formed in unclear if T is void this case http://cppreference.com 2024.06.10 std::any_cast(3)
NAME | Synopsis | Parameters | Return value | Exceptions | Example | Possible output:
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