为啥不能用cout<<a+b;?

#include <bits/stdc++.h>
using namespace std;

int main(){
    //1、存储输入
    string s1, s2;
    int a[205] = {0}, b[205] = {0}, c[205] = {0}; //a:加数 b:被加数 c:和
    cin >> s1 >> s2;
	int len1 = s1.size(), len2 = s2.size();
	//2、字符转为数字，倒序存储至整型数组中 
	for (int i = 0; i <=len1-1; i++)	a[len1-1-i] = s1[i] - '0';
	for (int i = 0; i <= len2-1; i++)	b[len2-1-i] = s2[i] - '0';
	//3、从个位（下标0的位置）开始做加法运算 
	int len3=max(len1,len2),jin=0;
	for(int i=0;i<=len3-1;i++){
	    c[i]=a[i]+b[i]+jin;
	    jin=c[i]/10;
	    c[i]%=10;
	}
	if(jin>0)
	c[len3++]=jin;
	
	
	//4、从高位向低位输出
    for (int i = len3-1; i >= 0; i--)	cout << c[i];
    return 0;
}