C++错误:未定义的符号

我的代码的目的是能够在文本中的某个地方输入一个带有空白-n的文件。假设将 blank-N 更改为用户提示的代码，例如 （blank-N） =noun 并要求用户输入名词，但由于某种原因我收到该错误所以这是我的代码：

    //Author:
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#include <iostream>
#include <iomanip>
#include <fstream>
#include <string>
using std::string;
using namespace std;
string getWord(ifstream &Infile, char &ch);
void getFilename(ifstream &Infile);
void getOutfile(ofstream &Outfile);
void putWord(ostream &Outfile,string word, char space);
string replacement(string word,std::string);
void printprompt(int i);
int main()
{
  ifstream filename;
  ofstream outfile;
  string file;
  string word;
  char ch;
  getFilename(filename);
  getOutfile(outfile);
  while(!filename.eof())
    {
      file =getWord(filename,ch);
      putWord(outfile,file,ch);
    }
  filename.close();
  outfile.close();
  return 0;
}
string getWord(ifstream &Infile, char &ch)
{
  string wrd = "";
  Infile.get(ch);
  while(ch !=' '&& ch != 'n' && !Infile.eof())
    {
      wrd.append(1,ch);
      Infile.get(ch);
    }
  return wrd;
}
void getFilename(ifstream &Infile)
{
  string filename;
  cout<<"File for input: ";
  cin >>filename;
  Infile.open(filename.c_str());
  if (Infile.fail())
    {
      cout << "Cannot open"<<filename<<endl;
      exit(0);
    }
}
void getOutfile(ofstream &Outfile)
{
  string filename;
  cout<<"file for output: ";
  cin>>filename;
  Outfile.open(filename.c_str());
  if(Outfile.fail())
    {
      cout<< "Cannot open"<<Outfile<<endl;
      exit(0);
    }
}
void putWord(ofstream &Outfile,string word, char c)
{
  Outfile << word;
  if (c =='n')
    Outfile<<endl;
  else
    Outfile<<" ";
}
string replacement(string word)
{
  string key[5]={"blank-N", "blank-A","blank-V","blank-P","blank-D"};
  for(int i =0; i<5;i++)
    {
      if(word.compare(key[i]))
        {
          printprompt(i);
          cin>>word;
        }
      return word;
    }
}
void printprompt(int i)
{
  switch(i)
    {
    case 0:
      cout<<"Please enter a noun";
      break;
    case 1:
      cout<<"Please enter an adjective";
      break;
    case 2:
      cout<<"Please enter a verb";
      break;
    case 3:
      cout<<"Please enter a place";
      break;
  default:;
    }
}

我似乎收到此错误：

    Undefined                       first referenced
 symbol                             in file
putWord(std::basic_ostream<char, std::char_traits<char> >&, std::basic_string<char, std::char_traits<char>, std::allocator<char> >, char)/var/tmp//ccdj0m4f.o
ld: fatal: Symbol referencing errors. No output written to a.out
collect2: ld returned 1 exit status

我不知道如何防止此错误发生。