std::is_convertible, std::is_nothrow_convertible

< cpp‎ | types
Metaprogramming library
Type traits
Type categories
Type properties
(C++11)(until C++20*)
(C++11)(deprecated in C++20)
Type trait constants
Supported operations
Relationships and property queries

Type modifications
Type transformations
(C++11)(deprecated in C++23)
(C++11)(deprecated in C++23)

(C++11)(until C++20*)(C++17)
Compile-time rational arithmetic
Compile-time integer sequences
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.


[edit] 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

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)
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{}
    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)
> {};
is_nothrow_convertible (2)
template<class From, class To>
struct is_nothrow_convertible : std::conjunction<std::is_void<From>, std::is_void<To>> {};
template<class From, class To>
            { std::declval<void(&)(To) noexcept>()(std::declval<From>()) } noexcept;
struct is_nothrow_convertible<From, To> : std::true_type {};

[edit] 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 Feature
__cpp_lib_is_nothrow_convertible 201806L (C++20) std::is_nothrow_convertible

[edit] Example

#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;
            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';


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

[edit] See also

checks if a type is derived from the other type
(class template) [edit]
checks if a type is a pointer-interconvertible (initial) base of another type
(class template) [edit]
checks if objects of a type are pointer-interconvertible with the specified subobject of that type
(function template) [edit]
specifies that a type is implicitly convertible to another type
(concept) [edit]