对C++ Primer的10.3.9单词转换的思考

#include <iostream>
#include <string>
#include <map>
#include <fstream>
#include <sstream>

using namespace std;
ifstream& open_file(ifstream&,const string&);

int main(int argc, char **argv)
{
    map<string, string> trans_map;
    string key, value;
    if (argc!= 3)
        throw runtime_error("wrong number of arguments");
    ifstream map_file;
    if (!open_file(map_file, argv[1]))
        throw runtime_error("no transformation file");
    while (map_file>>key>>value)
        trans_map.insert(make_pair(key, value));
    ifstream input;
    if (!open_file(input, argv[2]))
        throw runtime_error("no input file");
    string line; 
    while (getline(input, line))
    {
        istringstream stream(line); 
        string word;
        bool firstword = true;
        while (stream >> word) {
            map<string, string>::const_iterator map_it =trans_map.find(word);
            if (map_it != trans_map.end())
                word = map_it->second;
            if (firstword)
                firstword = false;
            else
                cout << " ";
            cout << word;
        }
        cout << endl; 
    }
    return 0;
}