C ++类中的运算符重载函数

Question

This is my standalone operator overloading function.

#include <fstream>
#include <iostream>
#include <sstream>
#include <string>
#include <vector>
using namespace std;

typedef vector <string> record_t;
typedef vector <record_t> data_t;

istream& operator >> ( istream& ins, record_t& record )
  {

  record.clear();
  string line;
  getline( ins, line );
  stringstream ss( line );
  string field;
  while (getline( ss, field, '\t' ))
    {
    stringstream fs( field );
    string f("");  // (default value is 0.0)
    fs >> f;
    record.push_back( f );
    }
  return ins;
  }

istream& operator >> ( istream& ins, data_t& data )
  {

  data.clear();
  record_t record;
  while (ins >> record)
    {
    data.push_back( record );
    }
  return ins;  
  }

int main()
  {
  // Here is the data we want.
  data_t data;

  ifstream infile( "split.idx" );
  infile >> data;

  if (!infile.eof())
    {
    cout << "Fooey!\n";
    return 1;
    }

  infile.close();

  // Otherwise, list some basic information about the file.
  cout << "Your CSV file contains " << data.size() << " records.\n";

  return 0;
  }

Here I've two Operator Overloading functions.

istream& operator >> ( istream& ins, record_t& record )
istream& operator >> ( istream& ins, data_t& data )

Now I wish to write the function in a Class format. So I declared the typedef data and record variables in Header File called ("Headers.h")

typedef vector <string> record_t;
typedef vector <record_t> data_t;

Now I wrote a class. FileHandler1.h

#ifndef _FILEHANDLER
#define _FILEHANDLER
#endif

class FileHandler1
{

    public : 


        istream& operator >> ( istream& ins, record_t& record );

        istream& operator >> ( istream& ins, data_t& data );

};

FileHandler1.cpp

#include "Headers.h"
#include "FileHandler1.h"
istream& FileHandler1:: operator >> ( istream& ins, record_t& record )
{
//Same set of code I posted initially 
}

istream& FileHandler1:: operator >> ( istream& ins, data_t& data )
{
// Same set of code  
}

Now I got below error.

error: 'std::istream& FileHandler1::operator>>(std::istream&, record_t&)' must take exactly one argument

How can I fix this bug ?

Answer 1

A binary operator, such as operator>> , can be implemented as either a non-member function or a member function. When it is implemented as a non-member function, it takes two arguments, let's say T and U . When it is implemented as a member function, it takes one argument - just U - and the left operand is the type of the class it is a member of.

That is, doing t >> u , where operator>> is a member of T , calls t.operator>>(u) .

Here, you're trying to make operator>> a member of FileHandler1 , even though the left operand of >> is not of that type. If you were allowed, the operator would have to be a member of istream , but that's not up to you because it's a standard type. You have to implement these functions as non-member functions.

If you do have an operator>> member of FileHandler1 , it will have to be declared with a single argument:

FileHandler1& operator >> ( U& right_operand );

And then you must use it with a FileHandler1 object as the left operand:

FileHandler1 fh;
U u;
fh >> u; // Equivalent to fh.operator>>(u)

Answer 2

The input operators in classes is for when a class instance is the input stream. Eg in your case:

FileHandler1 fh1;
fh1 >> something;

The above translates into:

fh1.operator>>(something);

This is the reason that member input operator only takes a single argument, which is what the error message tells you.

This doesn't seem to be what you want, so you have to keep the input operators as free-standing functions.