类中的结构体

class Class1 {
private:
struct struct1{
double x;
double y;
};
public:
double function(struct1 s1) {
return s1.x;
}
};

class Class2 {
public:
double function() {
return this->x;
}
private:
double x;
double y;
};

class Class3 {
public:
    double function(struct1 s1) {
        return s1.x;
    }//提示struct1未被定义
private:
    struct struct1 {
        double x;
        double y;
    };
};

先定义结构体，后在方法中使用结构体