编译运行程序遇到这样的问题：
terminate called after throwing an instance of ‘std:: out_of_range’ what(): basic_string::substr
内存读取越界。
解释1：
const std::string sTest( “test” );
sTest.substr( 0, 10 );
will raise the same exception, since you ask 10 characters, but only 5 ( sTest.length()) ) are available.
定义的sTest只有5个字符，要去sTest的字串（0，10），显然错误。
解释2：
std::string s(“foo”);
s.substr(5,1); //the length of the string is 3, 5 is out of bounds
同样会报错，out_of_range