This is a C++20 feature that allows aggregate initialization through the standard constructor syntax, rather than the typical braced-list initialization syntax. (Note that this only works if the parameters cannot be used in a valid call to a default or copy/move constructor. If they could, that would be called instead of performing aggregate initialization.) According … Read more
So what we have here is aggregate initialization covered in section 8.5.1 of the draft C++ standard and it says: An aggregate is an array or a class […] and: When an aggregate is initialized by an initializer list, as specified in 8.5.4, the elements of the initializer list are taken as initializers for the … Read more
I ran into this breaking change when I used GCC. The compiler printed an error for code like this: void foo(const unsigned long long &i) { unsigned int a[2] = {i & 0xFFFFFFFF, i >> 32}; } In function void foo(const long long unsigned int&): error: narrowing conversion of (((long long unsigned int)i) & 4294967295ull) … Read more
When viewing things this way it is easy to say there is complete and utter chaos in the way an object is initialized. The big difference comes from the type of foo: if it is an aggregate type or not. It is an aggregate if it has: no user-provided constructors (a deleted or defaulted function … Read more
How can I do what I want to do (that is, initialize an array in a constructor (not assigning elements in the body)). Is it even possible? Yes. It’s using a struct that contains an array. You say you already know about that, but then I don’t understand the question. That way, you do initialize … Read more
The trick is in C++14 8.4.2/5 [dcl.fct.def.default]: … A function is user-provided if it is user-declared and not explicitly defaulted or deleted on its first declaration. … Which means that C‘s default constructor is actually not user-provided, because it was explicitly defaulted on its first declaration. As such, C has no user-provided constructors and is … Read more
In C++11 having in-class member initializers makes the struct/class not an aggregate — this was changed in C++14, however. This is something I found surprising when I first ran into it, the rationale for this restriction is that in-class initializers are pretty similar to a user defined constructor but the counter argument is that no … Read more