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) |
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.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.
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) |
value
[static] | true if From is convertible to To , false otherwise (public static member constant) |
operator bool | converts the object to bool , returns value (public member function) |
operator()
(C++14) | returns value (public member function) |
Type | Definition |
---|---|
value_type | bool |
type | std::integral_constant<bool, value> |
Gives well-defined results for reference types, void types, array types, and function types.
#include <iostream> #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; }; bool b2a = std::is_convertible<B*, A*>::value; bool a2b = std::is_convertible<A*, B*>::value; bool b2c = std::is_convertible<B*, C*>::value; bool d2c = std::is_convertible<D, C>::value; // A Perfect Forwarding constructor make the class 'convert' from everything bool everything2e = std::is_convertible<A, E>::value; //< B, C, D, etc std::cout << std::boolalpha; std::cout << b2a << '\n'; std::cout << a2b << '\n'; std::cout << b2c << '\n'; std::cout << d2c << '\n'; std::cout << '\n'; std::cout << everything2e << '\n'; }
Output:
true false false true true
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