error: passing ‘const std::string‘ as ‘this‘ argument of

error: passing ‘const std::string’ as ‘this’ argument of ‘std::basic_string<_CharT, _Traits, _Alloc>& std::basic_string<_CharT, _Traits, _Alloc>::operator=(const std::basic_string<_CharT, _Traits, _Alloc>&) [with _CharT = char, _Traits = std::char_traits, _Alloc = std::allocator]’ discards qualifiers

//回调函数 void DealWithResponseThread::deliveryMsg(const std::string& topic, const char * key, size_t key_len, const char* msg, size_t msg_len, int64_t msg_offset, int32_t msg_partition) const { string s(msg); this->m_strCallBackJson = s; }

上面的程序在编译的时候，遇到上面的错误，原因是这个成员函数是只读函数（函数后面有const修饰），我对成员变量进行了赋值，所以报错。