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std::is_convertible, std::is_nothrow_convertible

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Defined in header <type_traits>
template< class From, class To >
struct is_convertible;
 (1) (since C++11)
template< class From, class To >
struct is_nothrow_convertible;
 (2) (since C++20)
1) If the imaginary function definition To test() { return std::declval<From>(); } is well-formed, (that is, either std::declval<From>() can be converted to To using implicit conversions, or both From and To are possibly cv-qualified void), provides the member constant value equal to true. Otherwise value is false. For the purposes of this check, the use of std::declval in the return statement is not considered an ODR-use.

If To is a reference type and a temporary object would be created when binding std::declval<From>() to To, the return statement in the imaginary function is considered well-formed, even though such binding is ill-formed in an actual function.

(since C++26)
Access checks are performed as if from a context unrelated to either type. Only the validity of the immediate context of the expression in the return statement (including conversions to the return type) is considered.
2) Same as (1), but the conversion is also noexcept.

If From or To is not a complete type, (possibly cv-qualified) void, or an array of unknown bound, the behavior is undefined.

If an instantiation of a template above depends, directly or indirectly, on an incomplete type, and that instantiation could yield a different result if that type were hypothetically completed, the behavior is undefined.

If the program adds specializations for any of the templates described on this page, the behavior is undefined.

Contents

[edit] Helper variable template

template< class From, class To >
constexpr bool is_convertible_v = is_convertible<From, To>::value;
 (since C++17)
template< class From, class To >
constexpr bool is_nothrow_convertible_v = is_nothrow_convertible<From, To>::value;
 (since C++20)
[edit]

Inherited from std::integral_constant

Member constants

value
[static]
 true if From is convertible to To, false otherwise
(public static member constant)

Member functions

operator bool
 converts the object to bool, returns value
(public member function)
operator()
(C++14)
 returns value
(public member function)

Member types

Type Definition
value_type bool
type std::integral_constant<bool, value>

[edit] Possible implementation

is_convertible (1) 
namespace detail
{
    template<class T>
    auto test_returnable(int) -> decltype(
        void(static_cast<T(*)()>(nullptr)), std::true_type{}
    );
    template<class>
    auto test_returnable(...) -> std::false_type;
 
    template<class From, class To>
    auto test_implicitly_convertible(int) -> decltype(
        void(std::declval<void(&)(To)>()(std::declval<From>())), std::true_type{}
    );
    template<class, class>
    auto test_implicitly_convertible(...) ->