Remember that std::unique_ptr may not always be managing an object -- either because it was created empty (using the default constructor or passing in a nullptr as the parameter), or because the resource it was managing got moved to another std::unique_ptr. So before we use either of ...
unique_ptr 类型智能指针在设计上最显著的特点是内部托管的指针一旦被创建就不能被任何形式的复制给另一个unique_ptr，只可以被移动给另一个unique_ptr。unique_ptr 没有拷贝构造函数，因此不能用于赋值。该指针最常用的情况是单例模式和编译防火墙的封装。
Case void f(); void f(int); void f(int, int); void f(double, double = 3.14); 匹配原...
The C++ standard calls the second template parameter D which is probably why it got mixed up. If you read the explanation for constructor 3 and 4 you see that it's the signatures under a) that apply because Deleter is void(*)(connection *) (as you said) which is not a reference ...