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std::any_cast

From cppreference.com
< cpp‎ | utility‎ | any
 
 
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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.

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.

Contents

[edit] Parameters

operand - target any object

[edit] 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.

[edit] Exceptions

1-3) Throws std::bad_any_cast if the typeid of the requested T does not match that of the contents of operand.

[edit] Example

#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