1. C++对象初始化方式

C++在不同的上下文会选择不同的对象初始化方式，这部分比较琐碎不适合记忆，本节仅阐述与Constructor设计相关且比较常见的情形。

如果需要了解某种初始化方式的细节，请查阅如下表格：

Direct Initialization link
Default Initialization link
Copy Initialization link
Reference Initialization link
Constant Initialization link
Zero Initialization link
Value Initialization link
Aggregate Initialization link
List Initialization link

1.1 Direct Initialization

效果：调用non-default constructor完成对象的构造和初始化。

// 1. 
T object(arg1, arg2, ...);
new T(arg1, arg2, ...)

// 2. Explicit type conversion
T(other)
T(arg1, arg2, ...)

// 3. static_cast
static_cast<T>(other)

// constructor initializer list
Class::Class() : member(arg1, arg2, ...) {...}

1.2 Default Initialization

效果：调用default constructor完成对象的构造和初始化。

// 1.
T object;
new T

// 2. 当class的non-static data member没有在constructor initializer list中被初始化时会首先
// 进行 Default Initialization，然后再执行constructor函数体。

1.3 Copy Initialization

效果：调用non-explicit constructor完成对象的构造和初始化。

// 1. 赋值符号
T object = other;

// 2. function, pass argument by value
func(other);

// 3. function, return object
return object;

2. 设计Constructor

Item 01: 显示声明使用或拒绝编译器默认提供的默认构造函数。

Item 02: Copy Initialization与Copy Constructor无关，与explicit修饰有关。

Item 03: Copy Constructor同时提供const和非const版本，Move Constructor不要加const修饰。

Item 04: 如果不希望类的实例进行Copy Initialization (可能会涉及一些隐式转换)，那么将Constructor声明为explicit。

通常，将除Copy Constructor和Move Constructor外的、单个参数的Constructor声明成为explicit是一个good practice，除非接受隐式转换！

看这里的讨论

If we would have started to design C++ from today on, there is a good chance, that all constructors and conversion functions were “explicit” by default, but a user could make the “implicit”. Alas, the time cannot be turned back and we have to live with the current state. This means that we have to be careful in regard to implicit constructors (except for the copy/move constructor). It is a safer rule to make constructors explicit, where some form of conversion is involved (i.e. any constructor with an argument type U different from the actual type T).

Item 05: …