Namespaces
Variants
Views
Actions

std::is_trivially_copyable

From cppreference.com
< cpp‎ | types
 
 
 
Type support
Basic types
Fundamental types
Fixed width integer types (C++11)
Numeric limits
C numeric limits interface
Runtime type information
Type traits
Type categories
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
Type properties
(C++11)
(C++11)
(C++14)
(C++11)
is_trivially_copyable
(C++11)
(C++11)(until C++20)
(C++11)(deprecated in C++20)
(C++11)
Type trait constants
Metafunctions
(C++17)
Constant evaluation context
Supported operations
Relationships and property queries
Type modifications
(C++11)(C++11)(C++11)
Type transformations
(C++11)
(C++11)
(C++17)
(C++11)(until C++20)(C++17)
 
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 cv-qualified).

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

Contents

[edit] Template parameters

T - a type to check

[edit] 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>

[edit] 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.

[edit] Example

#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

[edit] See also

checks if a type is trivial
(class template) [edit]