# std::is_convertible, std::is_nothrow_convertible

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< cpp‎ | types

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.
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`.

`From` and `To` shall each be a complete type, (possibly cv-qualified) void, or an array of unknown bound. Otherwise, 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.

The behavior of a program that adds specializations for any of the templates described on this page is undefined.

### Helper variable template

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

## 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>

### Possible implementation

(1) `is_convertible`
    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(...) -> std::false_type;
    } // namespace detail

    template<class From, class To>
    struct is_convertible : std::integral_constant<bool,
        (decltype(detail::test_returnable<To>(0))::value &&
         decltype(detail::test_implicitly_convertible<From, To>(0))::value) ||
        (std::is_void<From>::value && std::is_void<To>::value)
    > {};
(2) `is_nothrow_convertible`
    template<class From, class To>
    struct is_nothrow_convertible : std::conjunction<std::is_void<From>, std::is_void<To>> {};

    template<class From, class To>
        requires
            requires
            {
                static_cast<To(*)()>(nullptr);
                { std::declval<void(&)(To) noexcept>()(std::declval<From>()) } noexcept;
            }
    struct is_nothrow_convertible<From, To> : std::true_type {};

### Notes

Gives well-defined results for reference types, void types, array types, and function types.

Currently the standard has not specified whether the destruction of the object produced by the conversion (either a result object or a temporary bound to a reference) is considered as a part of the conversion. This is LWG issue 3400.

All known implementations treat the destruction as a part of the conversion, as proposed in P0758R1.

Feature-test macro | Value | Std | Comment
--- | --- | --- | ---
`__cpp_lib_is_nothrow_convertible` | 201806L | (C++20) | `std::is_nothrow_convertible`

### Example

Run this code

    #include <iomanip>
    #include <iostream>
    #include <string>
    #include <string_view>
    #include <type_traits>

    class E { public: template<class T> E(T&&) {} };

    int main()
    {
        class A {};
        class B : public A {};
        class C {};
        class D { public: operator C() { return c; } C c; };

        std::cout
            << std::boolalpha
            << std::is_convertible_v<B*, A*> << ' ' // true
            << std::is_convertible_v<A*, B*> << ' ' // false
            << std::is_convertible_v<D, C> << ' '   // true
            << std::is_convertible_v<B*, C*> << ' ' // false
            // Note that the Perfect Forwarding constructor makes the class E be
            // "convertible" from everything. So, A is replaceable by B, C, D..:
            << std::is_convertible_v<A, E> << ' ';  // true

        using std::operator "" s, std::operator "" sv;

        auto stringify = []<typename T>(T x)
        {
            if constexpr (std::is_convertible_v<T, std::string> or
                          std::is_convertible_v<T, std::string_view>)
                return x;
            else
                return std::to_string(x);
        };

        const char* three = "three";

        std::cout
            << std::is_convertible_v<std::string_view, std::string> << ' ' // false
            << std::is_convertible_v<std::string, std::string_view> << ' ' // true
            << std::quoted(stringify("one"s)) << ' '
            << std::quoted(stringify("two"sv)) << ' '
            << std::quoted(stringify(three)) << ' '
            << std::quoted(stringify(42)) << ' '
            << std::quoted(stringify(42.0)) << '\n';
    }

Output:

    true false true false true false true "one" "two" "three" "42" "42.000000"

### See also

is_base_of (C++11) | checks if a type is derived from the other type (class template)
is_pointer_interconvertible_base_of (C++20) | checks if a type is a pointer-interconvertible (initial) base of another type (class template)
is_pointer_interconvertible_with_class (C++20) | checks if objects of a type are pointer-interconvertible with the specified subobject of that type (function template)
convertible_to (C++20) | specifies that a type is implicitly convertible to another type (concept)

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