用正则表达式就几行程序而已：

#include <iostream>
#include <regex>
int main() {
    std::regex r("\\[fullText\\](.*)\\[rating\\]");
    std::string l;
    while(std::cin) {
        std::getline(std::cin, l);
        std::cout << std::regex_replace(l, r, "$1\n");
    }
}

如果你不纠结一定要用C++，那可以更短：

perl -pe 's/\[fullText\](.*)\[rating\]/$1/g'

逻辑很简单，知道一点字符串操作和文件操作就好。下面的代码可以实现你的要求，没有考虑异常处理，也没有过多考虑效率，需要的话你自己改改就好

#include <iostream>
#include <fstream>
#include <string>

using namespace std;

class Solution {
 public:
  int ProcessFile(const string &src_file, 
                  const string &dest_file,
                  const string &head, 
                  const string &end) {
    ifstream input(src_file.c_str(), ifstream::in);
    if (!input) {
      return -1;
    }
    ofstream output(dest_file.c_str(), ofstream::out);
    if (!output) {
      return -1;
    }
    string line;
    string ::size_type head_len = head.length();
    while(getline(input, line)) {
      string::size_type head_pos = line.find(head, 0);
      string::size_type end_pos = line.find(end, head_pos + head_len);
      output << line.substr(head_pos + head_len, 
                            end_pos - head_pos - head_len) << ' ';
    }
    input.close();
    output.close();
    return 0;
  }
};
int main() {
  string src_file = "input.txt", dest_file = "output.txt";
  string head_name = "[fullText]", end_name = "[rating]";
  Solution sln;
  if (sln.ProcessFile(src_file, dest_file, head_name, end_name) < 0) {
    cout << "Fail..." << endl;
  } else {
    cout << "Success..." << endl;
  }
  return 0;
}