Initializing the value of the POD structure is constexpr?

Question

Consider the structure:

struct mystruct { }; 

Is it true that this is always true:

 constexpr mystruct mystructInstance = mystruct(); 

i.e. what initialization of the POD value is equal to constexpr ? Similarly, how to determine if a structure is defined:

 struct mystruct { ~mystruct(); }; 

Finally, how about this:

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

I did not declare ctr as constexpr, however, are there any implicit deduction rules that guarantee this?

Answer 1

Requirements for constexpr variables:

The constexpr variable must satisfy the following requirements:

• its type must be LiteralType.
• it must be immediately created or assigned a value.
• constructor parameters or assigned value should contain only literals, constant variables and functions.
• the constructor used to construct the object (implicit or explicit) must satisfy the requirements of the constexpr constructor. In the case of an explicit constructor, it must have constexpr.

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Given your 3 structures:

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

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mystruct_1 is LiteralType . So, the following is true and compiled:

 constexpr mystruct_1 mystructInstance_1 = mystruct_1(); 

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mystruct_2 not a LiteralType , since it has a nontrivial destructor. Therefore, the following is not valid and will not compile:

 constexpr mystruct_2 mystructInstance_2 = mystruct_2(); 

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The same applies to mystruct_3 , moreover, it is not aggregate and does not provide a constexpr constructor. Therefore, the following is not valid and will not compile:

 constexpr mystruct_3 mystructInstance_3 = mystruct_3(); 

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You can also view descriptive error messages in this online demo .