Category of classes and struct in C++14 or above

More detail post at: https://learn.microsoft.com/en-us/cpp/cpp/trivial-standard-layout-and-pod-types?view=msvc-170

Term:

Layout: how the members of an object of class, struct or union type are arranged in memory.

Starting from c++14, class and struct has been categorized into three different types, which are Trivial type, Standard Layout type, and POD(Plain Old Data) type. Categorizing the type helped compiler, c++ program and metaprograms to reason about the suitability of any given type for operations that depend on a particular memory layout.

Standar library has function templates which are is_trivial<T>, is_standard_layout<T>, is_pod<T> for identifying the class type.

Trivial type

Trivial types have a trivial default constructor, trivial copy constructor, trivial copy assignment operator and trivial destructor. In each case, trivial means the constructor/operator/destructor is not user-provided and belongs to a class that has

1. no virtual functions or virtual base classes,
2. no base classes with a corresponding non-trivial constructor/operator/destructor
3. no data members of class type with a corresponding non-trivial constructor/operator/destructor

Standard layout types

Standard-layout types can have user-defined special member functions. In addition, standard layout types have these characteristics:

• no virtual functions or virtual base classes
• all non-static data members have the same access control
• all non-static members of class type are standard-layout
• any base classes are standard-layout
• has no base classes of the same type as the first non-static data member.
• meets one of these conditions:
  • no non-static data member in the most-derived class and no more than one base class with non-static data members, or
  • has no base classes with non-static data members

POD types

class or struct is POD type when it is both trivial and standard-layout.

Sample Example from MS

#include <type_traits>
#include <iostream>

using namespace std;

struct B
{
protected:
   virtual void Foo() {}
};

// Neither trivial nor standard-layout
struct A : B
{
   int a;
   int b;
   void Foo() override {} // Virtual function
};

// Trivial but not standard-layout
struct C
{
   int a;
private:
   int b;   // Different access control
};

// Standard-layout but not trivial
struct D
{
   int a;
   int b;
   D() {} //User-defined constructor
};

struct POD
{
   int a;
   int b;
};

Literal Type(from MS)

Whose layout can be determined at compiled type 1. void 2. scalar types 3. references 4. Arrays of void, scalar types or reference 5. A class that has a trivial destructor, and one or more constexpr constructors that are not move or copy constructors. Additionally, all its non-static data members and base classes must be literal types and not volatile.