链式调用中，是否调用了复制赋值？

Question

我想了解何时调用复制构造函数或赋值。假设如下：

class Foo {
public:
   Foo()
   {
      cout << "foo constructor " << this << endl;
      test_ = 44;
   }

   Foo(const Foo& f)
   {
      cout << "foo copy constructor " << this << endl;
      test_ = 55;
   }

   Foo& operator=(const Foo& other)
   {
      cout << "foo copy assignment " << this << endl;
      return *this;
   }

   Foo& operator=(Foo& other)
   {
      cout << "foo copy non-const assignment " << this << endl;
      return *this;
   }

   ~Foo() { cout << "foo destructor  " << this << endl; };

   int bar() const {
      cout << "bar call  " << this << endl;
      return 6;
   }

   int test_;
};


class Model {
public:
   Model()
   {
      foo_ = std::make_unique<Foo>();
   }

   Foo get_foo()
   {
      return *foo_;
   }

   std::unique_ptr<Foo> foo_;
};


int main() {
   cout << "start test 1" << endl;
   {
      Model model;
      Foo foo1 = model.get_foo();
      foo1.bar();
   }
   cout << "end test 1" << endl << endl;


   cout << "start test 2" << endl;
   {
      Model model;
      const Foo& foo2 = model.get_foo();
      foo2.bar();
   }
   cout << "end test 2" << endl;


   cout << "start test 3" << endl;
   {
      Model model;
      model.get_foo().bar();
   }
   cout << "end test 3" << endl;
   return 0;
}

最后提供输出。

1. 为什么测试1中没有调用复制赋值而调用了复制构造函数？
2. 为什么在测试 2 和 3 中调用复制构造函数？
3. 测试 3 中关于

   get_foo().bar()

   的具体步骤是什么？

我希望您能在代码中注意到任何类型的信息/问题（例如 get_foo() 取消引用唯一指针这一事实）。

start test 1
foo constructor 000001D4EF9B6720
foo copy constructor 00000058BED6FA94
bar call  00000058BED6FA94
foo destructor  00000058BED6FA94
foo destructor  000001D4EF9B6720
end test 1

start test 2
foo constructor 000001D4EF9B6F60
foo copy constructor 00000058BED6FAF4
bar call  00000058BED6FAF4
foo destructor  00000058BED6FAF4
foo destructor  000001D4EF9B6F60
end test 2

start test 3
foo constructor 000001D4EF9B6F60
foo copy constructor 00000058BED6FBF4
bar call  00000058BED6FBF4
foo destructor  00000058BED6FBF4
foo destructor  000001D4EF9B6F60
end test 3