what happens if I pass a struct to a vararg function?

Question

void f(int count, ...){
    //whatever
}

struct somestruct{
    size_t a, b, c;
};

int main() {
    somestruct s;
    f(1, s);    //what is actually passed?
}

Is the entire struct copied and passed on the stack? If so are copy constructors called? Is the pointer passed? Is this safe?

Answer 1

Yes, if you pass an lvalue, the lvalue to rvalue conversion will be done, which means calling the copy constructor to copy the object into a new copy and passing that as an argument.

Answer 2

void f(...) is using bit-wised copy. No default constructor or copy constructor will be generated for your somestruct as it only has C++ build-in types.

Is this safe?

Yes, this is perfectly safe.

I'll refer you to 'Inside C++ Object Model' chapter 2 The Semantics of Constructors