C++有参构造函数的调用方法

#include <iostream>
using namespace std;

class A {
   
   
public:
protected:
private:
};

struct C {
   
   

};

class B {
   
   
public:
protected:
private:
    A a;
    int a2;
};

void main() {
   
   
    printf("A:%d \n"<