std::any_cast (3) - Linux Manuals

std::any_cast: std::any_cast

NAME

std::any_cast - std::any_cast

Synopsis

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.

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 requested 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 <string>
  #include <iostream>
  #include <any>
  #include <utility>

  int main()
  {
      // simple example

      auto a = std::any(12);

      std::cout << std::any_cast<int>(a) << '\n';

      try {
          std::cout << std::any_cast<std::string>(a) << '\n';
      }
      catch(const std::bad_any_cast& e) {
          std::cout << e.what() << '\n';
      }

      // pointer example

      if (int* i = std::any_cast<int>(&a)) {
         std::cout << "a is int: " << *i << '\n';
      } else if (std::string* s = std::any_cast<std::string>(&a)) {
         std::cout << "a is std::string: " << *s << '\n';
      } else {
         std::cout << "a is another type or unset\n";
      }

      // advanced example

      a = std::string("hello");

      auto& ra = std::any_cast<std::string&>(a); //< reference
      ra[1] = 'o';

      std::cout << "a: "
          << std::any_cast<const std::string&>(a) << '\n'; //< const reference

      auto b = std::any_cast<std::string&&>(std::move(a)); //< rvalue reference

      // Note: 'b' is a move-constructed std::string,
      // 'a' is left in valid but unspecified state

      std::cout << "a: " << *std::any_cast<std::string>(&a) //< pointer
          << "b: " << b << '\n';
  }

Possible output:

  12
  bad any_cast
  a is int: 12
  a: hollo
  a: b: hollo