Static_assert(!std::is_pod::value, 'Y should not be POD') Īdditionally, explicitly defaulting a constructor will make it constexpr if the implicit constructor would have been and will also give it the same exception specification that the implicit constructor would have had. Static_assert(!std::is_trivial::value, 'Y should not be trivial') Static_assert(std::is_pod::value, 'X should be POD') Static_assert(std::is_trivial::value, 'X should be trivial') §9/6 A trivial class is a class that has a trivial default constructor and §12.1/5 A default constructor is trivial if it is not user-provided and

§8.5.1/1 An aggregate is an array or a class with no user-provided constructors,

If you want your class to be an aggregate or a trivial type (or by transitivity, a POD type), then you need to use = default. Giving a user-defined constructor, even though it does nothing, makes the type not an aggregate and also not trivial. However, while both constructors will behave the same, providing an empty implementation does affect some properties of the class. The implicitly-defined default constructor performs the set of initializations of the class that would be performed by a user-written default constructor for that class with no ctor-initializer (12.6.2) and an empty compound-statement. §12.1/6 A default constructor that is defaulted and not defined as deleted is implicitly defined when it is odr-used to create an object of its class type or when it is explicitly defaulted after its first declaration. A defaulted default constructor is specifically defined as being the same as a user-defined default constructor with no initialization list and an empty compound statement.

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