You can’t have your cake and eat it too. If the object has a constructor it is no longer an aggregate, and only aggregates can be initialized with designated initializers. You can’t use constructors for arbitrary initialization logic with aggregates. Are we toasted though? No, because there’s the “named constructor” idiom. It’s essentially just a … 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
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
Here is a trick which triggers a linker error if a required initializer is missing: struct init_required_t { template <class T> operator T() const; // Left undefined } static const init_required; Usage: struct Foo { int bar = init_required; }; int main() { Foo f; } Outcome: /tmp/ccxwN7Pn.o: In function `Foo::Foo()’: prog.cc:(.text._ZN3FooC2Ev[_ZN3FooC5Ev]+0x12): undefined reference to … Read more
Brace elision applies, but not in C++11. In C++14, they will apply because of http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1270 . If you are lucky, Clang will backport that to their C++11 mode (let’s hope they will!).
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
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
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
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