为什么s3在拼接之前能够输出但是拼接之后不能输出

Question

串的基本操作实现
为什么s3在拼接之前能够输出但是拼接之后不能输出
必须要放在数组里吗

#include <iostream>

using namespace std;
int strlen(char *s)
{
    char *p=s;
    int len=0;
    while(*p!='\0')
    {
        p++;
        len++;
    }
    return len;
}
char *stract(char *s1,char *s2)
{
    char *p=s1,*q=s2;
    while(*p!='\0')
    {
        p++;
    }
    while(*q!='\0')
    {
        *p=*q;
        p++;q++;
    }
    *p='\0';
    return s1;
}
int strcmp(char *s1,char *s2)
{
    char *p=s1,*q=s2;
    while(*p!='\0'&&*q!='\0')
    {
        if(*p>*q)
            return 1;
        else if(*p<*q)
            return -1;
        else
            {p++;q++;}
    }
    if(*p!='\0'&&*q!='\0')
        return 0;
    if(*p!='\0')
        return 1;
    if(*q!='\0')
        return -1;
}

int main()
{
    char s1[20]="I love ",*s2="China!";
    cout<<"The length of s1:"<<endl;
    cout<<strlen(s1)<<endl;
    cout<<"The length of s2:"<<endl;
    cout<<strlen(s2)<<endl;
    cout<<"s1>s2?"<<endl;
    cout<<strcmp(s1,s2)<<endl;
    cout<<"s2>s1?"<<endl;
    cout<<strcmp(s2,s1)<<endl;
    cout<<"s1+s2:"<<endl;
    stract(s1,s2);
    for(int i=0;s1[i]!='\0';i++)
    {
        cout<<s1[i];
    }
    cout<<endl;
    char *s3="hello ",*s4="world";
    cout<<s3<<endl;
    stract(s3,s4);
    cout<<s3<<endl;
    return 0;

}