C/C++函数返回值问题

int fun(int a,int b){return a+b;}
int main(){
    int a=10;
    int b=10;
    int ret=sum(a,b);
}

typedef struct data{
    int x;
    int y;
}Data;

Data sum(Data a,Data b){
    Data c;
    c.x=a.x+b.x;
    c.y=a.y+b.y;
    return c;
}
int main(){
    Data a={1,2};
    Data b={3,4};
    Data ret=sum(a,b);
    return 0;
}

typedef strut{
    int a[20];
}Data;

Data sum(Data a,Data b){
    Data temp = {0};
    temp.a[0]=a.a[0]+b.a[0];
    return temp;
}

int main(){
    Data a = {0};
    Data b = {0};
    Data ret = sum(a,b);
    return 0;
}