I made some changes

Wrong, wrong, wrong. High precision addition is correct on other platforms. This is incorrect!!!

#include<bits/stdc++.h>

using namespace std;

struct stadd{

int sz[300],len;
void init(){
    memset(sz,0,sizeof(sz));
    len=0;
}
void input(){
    string s;
    cin>>s;
    len=s.size();
    for(int i=0;i<len;i++){
        sz[len-i-1]=s[i]-'0';
    }
}
void output(){
    bool x=true;
    for(int i=len-1;i>=0;i--){
        if(!x || sz[i]>0){
            cout<<sz[i];
            x=false;
        }
    }
}
}; void add(stadd a,stadd b,stadd *c){ c->init();

int m=max(a.len,b.len);
c->len=m;
int x=0;
for(int i=0;i<m;i++){
    c->sz[i]+=a.sz[i]+b.sz[i];
    x=c->sz[i]/10;
    c->sz[i]%=10;
    c->sz[i+1]=x;
}
if(x>0){
    c->len++;
}
} stadd operator +(stadd a,stadd b){

 stadd c;
  c.init();
   add(a,b,&c);
   return c;
} int main(){

stadd a,b,c;
a.init();
b.init();
c.init();
a.input();
b.input();

c=a+b;

c.output();
return 0;
}