# std::is_trivially_copyable

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< cpp‎ | types

Defined in header `<type_traits>` |  |
template< class T > struct is_trivially_copyable; |  | (since C++11)

If `T` is a TriviallyCopyable type, provides the member constant `value` equal true. For any other type, `value` is false.

The only trivially copyable types are scalar types, trivially copyable classes, and arrays of such types/classes (possibly const-qualified, but not volatile-qualified).

The behavior is undefined if std::remove_all_extents_t<T> is an incomplete type and not (possibly cv-qualified) void.

### Template parameters

T | - | a type to check

### Helper variable template

template< class T > inline constexpr bool is_trivially_copyable_v = is_trivially_copyable<T>::value; |  | (since C++17)

## Inherited from std::integral_constant

### Member constants

value [static] | true if `T` is a trivially copyable type , 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

Objects of trivially-copyable types are the only C++ objects that may be safely copied with std::memcpy or serialized to/from binary files with `std::ofstream::write()`/`std::ifstream::read()`.

In general, for any trivially copyable type `T` and an object `obj1` of `T`, the underlying bytes of `obj1` can be copied (e.g. by means of std::memcpy or std::memmove) into an array of `char`, `unsigned char` or `std::byte` or into `obj2`, a distinct object of `T`. Neither `obj1` nor `obj2` may be a potentially-overlapping subobject.

If the underlying bytes of `obj1` is copied into such an array, and then the resulting content is copied back into `obj1`, `obj1` will hold its original value. If the underlying bytes of `obj1` are copied into `obj2`, `obj2` will hold `obj1`'s value.

### Example

Run this code

    #include <iostream>
    #include <type_traits>

    struct A {
        int m;
    };

    struct B {
        B(B const&) {}
    };

    struct C {
        virtual void foo();
    };

    struct D {
        int m;

        D(D const&) = default; // -> trivially copyable
        D(int x): m(x+1) {}
    };

    int main()
    {
        std::cout << std::boolalpha;
        std::cout << std::is_trivially_copyable<A>::value << '\n';
        std::cout << std::is_trivially_copyable<B>::value << '\n';
        std::cout << std::is_trivially_copyable<C>::value << '\n';
        std::cout << std::is_trivially_copyable<D>::value << '\n';
    }

Output:

    true
    false
    false
    true

### See also

is_trivial (C++11) | checks if a type is trivial (class template)

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