求助！新手问问题！多谢！

#include <iostream>
#include<string>                           //加这个头文件
using namespace std;
int main()
{const char *cp1="a and b";
const char *cp2="are f";
size_t len=strlen(cp1)+strlen(cp2);
char *result_str=new char[len+1];
strcpy(result_str,cp1);
strcat(result_str,cp2);                     //去掉这一行
cout<<strcat(result_str,cp2)<<endl;
delete [] result_str;
    return 0;
}

谢谢各位的帮助！！