结构体初始化会调用构造函数

using namespace std;

class cls{
public:
    cls(){
        c = 5;
        cout << "cls init" << endl;
    };
    int c;
};

struct s{
    s(){
        cout << a << endl;
        cout << b.c << endl;
    };
    int a;
    cls b;
};

int main(){
    s s1;
}

结构体构造函数中未赋值所有的变量，其中class会调用该类的构造函数

using namespace std;

class cls{
public:
    cls(){
        c = 5;
        cout << "cls init" << endl;
    };
    int c;
};

struct s{
    s(int c): a(c){
        cout << a << endl;
        cout << b.c << endl;
    };
    int a;
    cls b;
};

int main(){
    s s1(4);
}