Value initialization of POD struct is a constexpr?

Question

Consider the struct:

struct mystruct { };

Is it true that this is always valid:

constexpr mystruct mystructInstance = mystruct();

i.e. that value initialization of POD is a constexpr? Similarly how about if the struct is defined to be:

struct mystruct { ~mystruct(); };

Finally, what about this:

struct mystruct { mystruct(); ~mystruct(); };

I haven't declared the ctr as constexpr, however are there any implicit deduction rules which guaratee this?

Answer 1

The requirements for constexpr variables are:

A constexpr variable must satisfy the following requirements:

• its type must be a LiteralType.
• it must be immediately constructed or assigned a value.
• the constructor parameters or the value to be assigned must contain only literal values, constexpr variables and functions.
• the constructor used to construct the object (either implicit or explicit) must satisfy the requirements of constexpr constructor. In the case of explicit constructor, it must have constexpr specified.

---

Given your 3 structs:

struct mystruct_1 { };
struct mystruct_2 { ~mystruct_2(); };
struct mystruct_3 { mystruct_3(); ~mystruct_3(); };

---

mystruct_1 is a LiteralType. So the following is valid and compiles:

constexpr mystruct_1 mystructInstance_1 = mystruct_1();

---

mystruct_2 is not a LiteralType since it has a non-trivial destructor. Therefore the following is invalid and fails to compile:

constexpr mystruct_2 mystructInstance_2 = mystruct_2();

---

The same applies for mystruct_3, additionally it is not an aggregate and does not provide a constexpr constructor. So the following is also invalid and fails to compile:

constexpr mystruct_3 mystructInstance_3 = mystruct_3();

---

You can also have a look at the descriptive error messages in this online demo.