Constructing elements of union - c++

Question

The following class outputs when a constructor is called:
class A {
public:
A() {
std::cout << "Default Constructor called at address:" << this << "!\n";
}
A(int val) : val_(val) {
std::cout << "Secondary Constructor called at address:" << this << "\n";
}
A(A&& other) : val_(std::move(other.val_)) {
std::cout << "Move constructor called from " << &other << " to " << this << "\n";
}
A& operator=(A&& other) {
std::cout << "Move assignment called from " << &other << " to " << this << "\n";
val_ = std::move(other.val_);
return *this;
}
private:
int val_;
};
This union allows me to skip the default constructor of A, which is desirable:
union MyUnion {
MyUnion() {}
A a;
};
My goal with main was to move-construct a. Instead it move-assigns the a.
int main(){
MyUnion my_union[2];
std::cout << "Entering for loop:\n";
for (int i = 0; i < 2; ++i){
my_union[i].a = std::move(A(i));
}
return 0;
}
An example output from the above main function is:
Entering for loop:
Secondary Constructor called at address:0x7ffcdd3553a8
Move assignment called from 0x7ffcdd3553a8 to 0x7ffcdd3553b0
Secondary Constructor called at address:0x7ffcdd3553a8
Move assignment called from 0x7ffcdd3553a8 to 0x7ffcdd3553b4
I wanted to move-construct, but I'm instead move-assigning. Move-assigning without move constructing is unsafe in general. The solution I came up with was to use std::construct_at. I also added a destructor. The full program with these modifications is below:
#include <iostream>
#include <memory>
#include <utility>
class A {
public:
A() {
std::cout << "Default Constructor called at address:" << this << "!\n";
}
A(int val) : val_(val) {
std::cout << "Secondary Constructor called at address:" << this << "\n";
}
A(A&& other) : val_(std::move(other.val_)) {
std::cout << "Move constructor called from " << &other << " to " << this << "\n";
}
A& operator=(A&& other) {
std::cout << "Move assignment called from " << &other << " to " << this << "\n";
val_ = std::move(other.val_);
return *this;
}
~A() {
std::cout << "Destructor called at " << this << "\n";
}
private:
int val_;
};
union MyUnion {
MyUnion() {
std::cout << "MyUnion Constructor called!\n";
}
void ConstructAt(A&& other){
std::construct_at(&a, std::forward<A>(other));
}
A a;
~MyUnion(){
std::cout << "MyUnion Destructor called!\n";
a.~A();
}
};
int main(){
MyUnion my_union[2];
std::cout << "\nEntering for loop:\n";
for (int i = 0; i < 2; ++i){
my_union[i].ConstructAt(std::move(A(i)));
}
std::cout << "\nExiting for loop:\n";
return 0;
}
This gives output:
MyUnion Constructor called!
MyUnion Constructor called!
Entering for loop:
Secondary Constructor called at address:0x7ffe38dd4838
Move constructor called from 0x7ffe38dd4838 to 0x7ffe38dd4840
Destructor called at 0x7ffe38dd4838
Secondary Constructor called at address:0x7ffe38dd4838
Move constructor called from 0x7ffe38dd4838 to 0x7ffe38dd4844
Destructor called at 0x7ffe38dd4838
Exiting for loop:
MyUnion Destructor called!
Destructor called at 0x7ffe38dd4844
MyUnion Destructor called!
Destructor called at 0x7ffe38dd4840
This appears to be what I want.
Question
Was this the correct way to call the move constructor? Am I using std::forward correctly? Is this the correct way to handle the destructor of a union?