c++ - Constexpr默认功能

Question

可以是constexpr的默认函数是隐式constexpr吗？

class c
{
public:
    constexpr /*<- necessary?*/ c() = default;
    constexpr /*<- necessary?*/ c(c const&) = default;
    constexpr /*<- necessary?*/ c(c &&) = default;
    constexpr /*<- necessary?*/ c& operator =(c const&) = default;
    constexpr /*<- necessary?*/ c& operator =(c &&) = default;
    ~c() = default;

private:
    int x = 0;
};

那么其他限定词，例如 noexcept呢？

Answer 1

是的，如果默认特殊成员满足constexpr成员的要求，则为constexpr。特别是，有关默认构造函数，请参见11.4.4.1/4:

A default constructor that is defaulted and not defined as deleted is implicitly defined... 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 and an empty compound-statement. ... If that user-written default constructor would satisfy the requirements of a constexpr constructor ([dcl.constexpr]), the implicitly-defined default constructor is constexpr. Before the defaulted default constructor for a class is implicitly defined, all the non-user-provided default constructors for its base classes and its non-static data members are implicitly defined. ...

其他特殊成员也有类似的措辞。