std::is_standard_layout - cppreference.com

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# std::is_standard_layout

From cppreference.com
< cpp‎ | types

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

If `T` is a standard layout type (that is, a scalar type, a standard-layout class, or an array of such type/class, possibly cv-qualified), provides the member constant `value` equal true. For any other type, `value` is false.

A standard-layout class is a class that satisfies `StandardLayoutType`.

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

### [edit] Template parameters

T | - | a type to check

### [edit] Helper variable template

template< class T > constexpr bool is_standard_layout_v = is_standard_layout<T>::value; |  | (since C++17)

[edit]

## Inherited from std::integral_constant

### Member constants

value [static] | true if `T` is a standard-layout 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

A pointer to a standard-layout class may be converted (with reinterpret_cast) to a pointer to its first non-static data member and vice versa.

If a standard-layout union holds two or more standard-layout structs, it is permitted to inspect the common initial part of them.

The macro offsetof can only be used with standard-layout classes.

### [edit] Example

Run this code

    #include <iostream>
    #include <type_traits>

    struct A {
        int m;
    };

    struct B {
        int m1;
    private:
        int m2;
    };

    struct C {
        virtual void foo();
    };

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

Output:

    true
    false
    false

### [edit] See also

is_trivially_copyable (C++11) | checks if a type is trivially copyable (class template) [edit]
is_pod (C++11) | checks if a type is plain-old data (POD) type (class template) [edit]
offsetof | byte offset from the beginning of a standard-layout type to specified member (function macro) [edit]
std::experimental::is_standard_layout_v (library fundamentals TS) | variable template alias of `std::is_standard_layout::value` (variable template) [edit]

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