聚合体及其初始化———指派初始化器，C++长大了，你应该知道指派初始化器

struct Point
{
  int x;
  int y;
};
void using_initial_list()
{
  //初始化变量
  auto channel_num{ 10 };
  
  //初始化聚合体
  Point p{ 10, 30 };
  std::vector<int> vec{ 1,2 };//contains two elements, 1 and 2;
}

//情形1
class People
{
public:
std::string m_name;
int m_age;
//People(std::string name, int age):m_name(name),m_age(age){}//非聚合体
//People(){}//非聚合体
};

//情形2
class Base {
public:
  Base(int i, int j){};
  int i =10;
  int j =30;
};

class People:public Base//非聚合体
{
public:
std::string m_name;
int m_age;
};


//情形3：
class People
{
public:
std::string m_name;
int m_age;
private:
 int math_score;//非聚合体
};

//情形4
class People
{
public:
std::string m_name;
int m_age;
virtual void worker(){};//非聚合体
};

struct AudioInfo
{
    int channel_num;
    int channel_layout;
    int sample_rate;
    int frame_cout;
    int bit_depth;
};
AudioInfo info1{1,1,48000,1024,16};//初始化列表
AudioInfo info2{ .channel_num = 1, .channel_layout {1}, .bit_depth=16,  };//指派初始化器

char cv[4] = {'a', 's', 'd', 'f', 0}; // 错误，多余元素个数
int x[] = {};                         // 错误，空的初始化器列表初始化未知边界的数组

union u { int a; const char* b; };
 
u a = {1};                   // OK：显式初始化成员 a
u b = {0, "asdf"};           // 错误：显式初始化两个成员
u c = {"asdf"};              // 错误：按元素声明顺序初始化，即不能以 "asdf" 初始化 int
 
// C++20 指派初始化器列表
u d = {.b = "asdf"};         // OK：可以显式初始化非开头元素
u e = {.a = 1, .b = "asdf"}; // 错误：显式初始化两个成员

u f{.a{10.0}};//错误，窄化转换

//example 1
struct S
{
  union 
  {
  int a;
  int b;
  };
  int c;
};
S s{.a{10}, .c{30}};

//example 2
struct A
{
    int x;
 
    struct B
    {
        int b1;
        int b2;
    } b;
};
 a = {1, {2, 3}}; // 以 1 初始化 a.x，以 2 初始化 a.b.b1，以 3 初始化 a.b.b2
 a = {1, {2, 3.0}}; // error, 存在窄化转换

//example 221
struct AudioInfo
{
    int channel_num;
    int channel_layout;
    int sample_rate;
    int frame_cout;
    int bit_depth;
};

AudioInfo info{ .channel_num = 1, .channel_layout {1}, .bit_depth=16,  };//sample_rate和frame_cout初始化为0

//example 222
struct AudioInfo
{
    int channel_num;
    int channel_layout;
    int sample_rate;
    int frame_cout;
    int bit_depth;
};//example 222
struct AudioInfo
{
    int channel_num;
    int channel_layout;
    int sample_rate;
    int frame_cout;
    int bit_depth;
};

AudioInfo info{ .channel_num = 1, .channel_layout {1}, .bit_depth=16.0,  };//error,double类型的16.0需要转换为int型，报错

//example 223
struct S
{
  union 
  {
    int a;
    int b;
  };
  int c;
};

S s{.a{10}, .c{30}};


union u { int a; const char* b; };
 
u f = {.b = "asdf"};         // OK：联合体的活跃成员是 b
u g = {.a = 1, .b = "asdf"}; // 错误：只可提供一个初始化器  

//example 224
struct C
{
    string str;
    int a = 2;
    int b= -1;
};
 
A{.b = 21}  //使用{}初始化str， 使用2初始化a，使用21初始化b