# std::is_destructible, std::is_trivially_destructible, std::is_nothrow_destructible

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< cpp‎ | types

Defined in header `<type_traits>` |  |
template< class T > struct is_destructible; | (1) | (since C++11)
template< class T > struct is_trivially_destructible; | (2) | (since C++11)
template< class T > struct is_nothrow_destructible; | (3) | (since C++11)

1) If `T` is a reference type, provides the member constant value equal to true.
If `T` is (possibly cv-qualified) void, a function type, or an array of unknown bound, value equals false.
If `T` is an object type, then, for the type `U` that is std::remove_all_extents <T>::type, if the expression std::declval <U&>().~U() is well-formed in unevaluated context, value equals true. Otherwise, value equals false.
2) Same as (1) and additionally std::remove_all_extents <T>::type is either a non-class type or a class type with a trivial destructor.
3) Same as (1), but the destructor is noexcept.

If `T` 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.

### Helper variable templates

template< class T > constexpr bool is_destructible_v = is_destructible<T>::value; |  | (since C++17)
template< class T > constexpr bool is_trivially_destructible_v = is_trivially_destructible<T>::value; |  | (since C++17)
template< class T > constexpr bool is_nothrow_destructible_v = is_nothrow_destructible<T>::value; |  | (since C++17)

## Inherited from std::integral_constant

### Member constants

value [static] | true if `T` is destructible, 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>

### Notes

Because the C++ program terminates if a destructor throws an exception during stack unwinding (which usually cannot be predicted), all practical destructors are non-throwing even if they are not declared noexcept. All destructors found in the C++ standard library are non-throwing.

Storage occupied by trivially destructible objects may be reused without calling the destructor.

### Possible implementation

is_destructible (1)

    // C++20 required
    template<typename t>
    struct is_destructible
        : std::integral_constant<bool, requires(t object) { object.~t(); }>
    {};

is_trivially_destructible (2)

    // Not real C++. Shall P2996 be approved, the following implementation will be available:
    template<typename t>
    struct is_trivially_destructible
         : std::integral_constant<bool, std::meta::type_is_trivially_destructible(^t)>
    {};

is_nothrow_destructible (3)

    // C++20 required
    template<typename t>
    struct is_nothrow_destructible
        : std::integral_constant<bool, requires(t object) { {object.~t()} noexcept; }>
    {};

### Example

Run this code

    #include <iostream>
    #include <string>
    #include <type_traits>

    struct Foo
    {
        std::string str;
        ~Foo() noexcept {};
    };

    struct Bar
    {
        ~Bar() = default;
    };

    static_assert(std::is_destructible<std::string>::value == true);
    static_assert(std::is_trivially_destructible_v<Foo> == false);
    static_assert(std::is_nothrow_destructible<Foo>() == true);
    static_assert(std::is_trivially_destructible<Bar>{} == true);

    int main() {}

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 2049 | C++11 | the specification was incompletable because of the imaginary wrapping struct | made complete

### See also

is_constructibleis_trivially_constructibleis_nothrow_constructible (C++11)(C++11)(C++11) | checks if a type has a constructor for specific arguments (class template)
has_virtual_destructor (C++11) | checks if a type has a virtual destructor (class template)
destructible (C++20) | specifies that an object of the type can be destroyed (concept)
destructor | releases claimed resources

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