C++ quiz. In what cases will class <code>A</code> have at mo

In what cases will class A have at most one constructor, taking into account the constructors automatically generated by the compiler? (code using this class may be arbitrary).

```
struct A {
  A(const A&) {}
};
```
```
struct A {};
```
```
struct A {
  A() {}
};
```
```
struct A {
  explicit A(int) {}
};
```

struct A {
  void operator=(const A&) {}
};

Explanation

The compiler can automatically generate two constructors - the default constructor and the copy constructor. The default constructor is only generated if there are no explicitly declared constructors in this class. A copy constructor is generated if there is an explicit declaration of this constructor. Therefore, a class can have exactly one constructor only if the copy constructor is explicitly declared.