C++编程法则365条一天一条（359）认识各种初始化术语

参考：
https://en.cppreference.com/w/cpp/language/copy_initialization
https://en.cppreference.com/w/cpp/language/default_initialization
https://en.cppreference.com/w/cpp/language/aggregate_initialization

Default initialization默认初始化

T object ;	(1)	
new T	(2)	

当未指定初始化器时，就叫默认初始化。

除了以上显示定义的场景之外，基类、非静态成员没有指定初始化器时，都属于默认初始化。

如果T是对象类型，那么将调用默认构造进行初始化，非对象类型将按照生命周期的不同走不同策略（例如堆栈变量，不做任何初始化操作，其值不确定；对于静态、线程、全局变量，初始化为0）。

需要注意的是，如果是const对象进行默认初始化，chus需要用户提供一个默认构造函数，编译器提供的默认构造不做数，否则编译报错：

default initialization of an object of const type 'const T1' without a user-provided default constructor

#include <string>struct T1 { int mem; };struct T2
{int mem;T2() { } // "mem" is not in the initializer list
};int n; // static non-class, a two-phase initialization is done:// 1) zero initialization initializes n to zero// 2) default initialization does nothing, leaving n being zeroint main()
{int n;            // non-class, the value is indeterminatestd::string s;    // class, calls default ctor, the value is "" (empty string)std::string a[2]; // array, default-initializes the elements, the value is {"", ""}
//  int& r;           // error: a reference
//  const int n;      // error: a const non-class
//  const T1 t1;      // error: const class with implicit default ctorT1 t1;            // class, calls implicit default ctorconst T2 t2;      // const class, calls the user-provided default ctor// t2.mem is default-initialized (to indeterminate value)
}

Copy initialization拷贝初始化

用一个对象初始化另一个对象，叫做拷贝初始化，不一定是拷贝构造，也可能是构造。

T object = other;	(1)	
T object = {other};	(2)	(until C++11)
f(other)	        (3)	
return other;	    (4)	
throw object;
catch (T object)    (5)	
T array[N] = {other-sequence};	(6)	

聚合初始化