std::is_standard_layout

 

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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 to 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.

The behavior of a program that adds specializations for is_standard_layout or is_standard_layout_v (since C++17) is undefined.

Template parameters

T

-

a type to check

Helper variable template

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

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>

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 is only guaranteed to be usable with standard-layout classes.

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

See also

is_trivially_copyable (C++11)	checks if a type is trivially copyable (class template)
is_pod (C++11)(deprecated in C++20)	checks if a type is a plain-old data (POD) type (class template)
offsetof	byte offset from the beginning of a standard-layout type to specified member (function macro)