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Defined in header <any> | ||
template<class T> T any_cast(const any& operand); | (1) | (since C++17) |
template<class T> T any_cast( any& operand); | (2) | (since C++17) |
template<class T> T any_cast( any&& operand); | (3) | (since C++17) |
template<class T> const T* any_cast(const any* operand)noexcept; | (4) | (since C++17) |
template<class T> T* any_cast( any* operand)noexcept; | (5) | (since C++17) |
Performs type-safe access to the contained object.
LetU bestd::remove_cv_t<std::remove_reference_t<T>>.
Contents |
| operand | - | targetany object |
T matches that of the contents ofoperand, a pointer to the value contained by operand, otherwise a null pointer.T does not match that of the contents ofoperand.#include <any>#include <iostream>#include <string>#include <type_traits>#include <utility> int main(){// Simple exampleauto 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(conststd::bad_any_cast& e){std::cout<<"2) "<< e.what()<<'\n';} // Pointer exampleif(int* i= std::any_cast<int>(&a1))std::cout<<"3) a1 is int: "<<*i<<'\n';elseif(std::string* s= std::any_cast<std::string>(&a1))std::cout<<"3) a1 is std::string: "<<*s<<'\n';elsestd::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<conststd::string&>(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 statestd::cout<<"5) a1.size(): "<< std::any_cast<std::string>(&a1)->size()// pointer<<'\n'<<"6) s1: "<< s1<<'\n';}
Possible output:
1) a1 is int: 122) bad any_cast3) a1 is int: 124) a1 is string: hollo5) a1.size(): 06) s1: hollo
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| LWG 3305 | C++17 | the behavior of overloads(4,5) was unclear ifT isvoid | the program ill-formed in this case |
