神奇bug,这是UB吗?

系统Win7旗舰版使用Dev-c++5.11 编译器32bit Release

在做P1229
写出来了这玩意

c="里面是随便打的"

逆天机房电脑截不了图

CPP

#include<iostream>
#include<cstring>
int n,x=1;
std::string a,b,c;
int main(){
	std::cin>>a>>b;
	n=a.size()-1;
	for(int i=0;i<n;++i){
		c=/*"u89asud8fu89sadu8f9usdi09fiosdof-0o-sdof[dfo0i09df9id8fjd]"+*/a[i]+a[i+1];std::cout<<c<<'\n';
		if(b.find(c))x<<=1;
	}printf("%d",x);
}

将c=/*"u89asud8fu89sadu8f9usdi09fiosdof-0o-sdof[dfo0i09df9id8fjd]"+*/a[i]+a[i+1];std::cout<<c<<'\n';中的/**/删掉后输出了这个：

TEXT

abc
cba
ic_string::at: __n (which is %zu) >= this->size() (which is %zu)
_string::at: __n (which is %zu) >= this->size() (which is %zu)
4
--------------------------------
Process exited after 0.4427 seconds with return value 0
请按任意键继续. . .

(abc 和 cba 是输入)

又写了一个：

CPP

#include<iostream>
#include<cstring>
int n,x=1;
std::string a,b,c;
int main(){
	std::cin>>a>>b;
	n=a.size()-1;
	for(int i=0;i<n;++i){
		c="8a9syd89ysad98shdfuh9s78hdfs78dfgy7gsdft67tdf"+a[i]+a[i+1];std::cout<<c<<'\n';
		if(b.find(c))x<<=1;
	}printf("%d",x);
}

TEXT

abc
cba
t: __n (which is %zu) >= this->size() (which is %zu)
 __n (which is %zu) >= this->size() (which is %zu)
4
--------------------------------
Process exited after 0.2081 seconds with return value 0
请按任意键继续. . .

在Linux上输出啥也没有。
求问为啥

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神奇bug,这是UB吗?