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1.1 为什么是构造函数？
构造函数
自定义构造函数
没有参数
自定义默认构造函数
构造函数默认实参均有定义
默认构造函数
一个参数
可以隐式转换构造函数
编译器合成默认构造函数
类只是一个抽象，需要将其实例化。
1.2构造函数定义一个或者多个，用于控制对象初始化的特殊成员函数。
1.3 构造函数声明构造函数可以初始化非static数据成员。构造函数声明满足三条件：
- 函数名等于类名
- 没有返回值
- 不能是const(扩展知识点：const成员函数)
构造函数之所以不能为const成员函数，是因为构造函数是需要初始化数据成员的，如果声明为const就没法通过this指针来初始化状态了，索性就把这种可能性扼杀了。
1.4 构造函数初始化构造函数初始化列表是用于初始化数据成员的。它位于构造函数声明与定义之间，列表可能只负责部分数据成员的初值给定，对于未给定的初值的成员
- 使用类内初值
- 若仍未初始化则默认初始化（扩展知识点：默认初始化）
类用户使用构造函数构建对象，根据参数不同选择不同的构造函数。
1.6 编译器合成的默认构造函数一个类没有自定义构造函数，编译器将会生成；声明了一个自定义构造函数，编译器则不会为你做任何构造函数，除非你用了default关键字
默认构造函数能初始化一些STL库，但是不能初始化内置类型和复合类型
注：内置类型就是基础的数据元素，如float int ，复合类型是数组指针之类的。
其他知识 类成员声明顺序应与初始化列表顺序一致注意：类中声明决定初始化顺序，初始化列表的顺序没有任何意义。
class X
{
int i;
int j;
public:
X(int val):j(val),i(j){};
}
类中的声明顺序是先i后j，构造函数初始化首先使用j初始化i，显然是错误的。
委托构造函数减少重复初始化列表委托构造函数用大白话说就是“利用一个比较完整的构造函数来完成一些，部分构造函数”
class A
{
int a;
int b;
int c;
int d;
string str;
public:
A(int p_a,int p_b,int p_c,int p_d,string str):a(p_a),b(p_b),c(p_c),d(p_d),str(p_str){{};//一个比较完整的构造函数（完成了所有初始化成员操作）
A():A(1,2,3,4,5,"");//默认构造函数
A(int a,int d):A(a,2,3,d,"apple");
}
