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std::is_convertible, std::is_nothrow_convertible - cppreference.com

template< class From, class To >
struct is_convertible;

template< class From, class To >
struct is_nothrow_convertible;

If the imaginary function definition

is well-formed, (that is, either

can be converted to

using

, or both

and

are possibly cv-qualified

), provides the member constant

equal to

. Otherwise

is

. For the purposes of this check, the use of

in the return statement is not considered an

.

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.

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.

template< class From, class To >
constexpr bool is_convertible_v = is_convertible<From, To>::value;

template< class From, class To >
constexpr bool is_nothrow_convertible_v = is_nothrow_convertible<From, To>::value;

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)
> {};

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 {};

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.

#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; };
 
    static_assert(std::is_convertible_v<B*, A*>);
    static_assert(!std::is_convertible_v<A*, B*>);
    static_assert(std::is_convertible_v<D, C>);
    static_assert(!std::is_convertible_v<B*, C*>);
    // Note that the Perfect Forwarding constructor makes the class E be
    // "convertible" from everything. So, A is replaceable by B, C, D..:
    static_assert(std::is_convertible_v<A, E>);
 
    static_assert(!std::is_convertible_v<std::string_view, std::string>);
    static_assert(std::is_convertible_v<std::string, std::string_view>);
 
    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);
    };
 
    using std::operator "" s, std::operator "" sv;
    const char* three = "three";
 
    std::cout << 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:

"one" "two" "three" "42" "42.000000"

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