C++, How to read data from file and insert in container: Map - c++

Question

I have a problem with fill container map with information . I read the information with operator>> but it gives me these errors in my second class CDealer. no operator ">>" matches these operands and binary">>": no operator found which takes a right-hand operand of type 'const CStock'(or these is no acceptable conversion)
I have three classes: CStock, CDealer who have map<CStock, pair<unsigned, double>> Stock; and CShop who have vector<CDealer*> Dealers;
I'll be very thankful if someone can help me.
This is my operators << and >> in CStock
ostream &operator << (ostream &toStream, const CStock &S){
toStream << "Name Stock: " << S.m_strName_stock << " Producer: " << S.m_strProducer <<
"Date: " << S.Date;
return toStream;
}
istream &operator >> (istream &fromStream, CStock &S)
{return fromStream >> S.m_strName_stock >> S.m_strProducer >> S.Date;}
This is my operators << and >> in CDealer
`
ostream &operator << (ostream &toStream, CDealer &D)
{
toStream << "Name Dealer: " << D.m_strName_Dealer << " Agent: " << D.m_strAgent <<
"Address: " << D.m_strAddress;
map<CStock, pair<unsigned, double>>::const_iterator it = D.Stock.begin();
while (it != D.Stock.end())
{
toStream << it->first <<it->second.first << it->second.second;
}
return toStream;
}
istream &operator >> (istream &fromStream, CDealer &D)
{
map<CStock, pair<unsigned, double>>::iterator it = D.Stock.begin();
fromStream >> D.m_strName_Dealer >> D.m_strAgent >> D.m_strAddress;
while (it!= D.Stock.end())
{fromStream >> it->first >> it->second.first >> it->second.second;}
return fromStream
}
And this is the constructor with parameter: file name and these tho operators <<, >>
CShop::CShop(const string &fname)
{
CDealer c;
fstream File(fname, ios::in);
if (File.is_open())
{
File >> m_strNameShop;
File >> m_strCity;
while (File>>c)
{
Dealers.push_back(new CDealer(c));
}
File.close();
}
else
throw "ERROR! ";
}
ostream &operator << (ostream &toStream, const CShop &S)
{
toStream << "Name Shop: " << S.m_strNameShop << " City: " << S.m_strCity;
vector<CDealer* >::const_iterator it = S.Dealers.begin();
while (it != S.Dealers.end())
{
CDealer* dealerPtr = *it++;
toStream << *dealerPtr<< endl;
}
return toStream;
}
istream &operator >> (istream &fromStream, CShop &D)
{
return fromStream >> D.m_strNameShop >> D.m_strCity;
}
And in the end main()
#include"CDealer.h"
#include"CShop.h"
#include<iostream>
#include<string>
#include <stdlib.h>
#include<vector>
using namespace std;
int main()
{
CShop SS1("data.txt");
cout << SS1;
system("pause");
return 0;
}

Answer 1

The implmentation of istream &operator >> (istream &fromStream, CDealer &D)
is poorly thought through.
The objective of the function is to read data from an input stream and flesh out the contents of the CDealer object. Normally the operator<< and operator>> functions work in tandem so that what you write using operator<< can be read back using operator>>.
From that point of view, even the operator<< function needs to revamped.
Here's one implementation that should work as long as there are no spaces in the string objects. If you have space in your string objects, the code needs be changed.
std::ostream& operator<<(std::ostream &toStream, CDealer cosnt& D)
{
// Write the name of the dealer, without any decorative text.
// Add a space character to the output so you can read the name back.
toStream << D.m_strName_Dealer << " ";
// Do the same for name of the agent and address.
toStream << D.m_strAgent << " ";
toStream << D.m_strAddress << " ";
// Write the number of items in Stock first.
// This will be necessary when reading the data.
toStream << D.Stock.size() << " ";
// Now write the Stock items, which spaces between each field you write.
map<CStock, pair<unsigned, double>>::const_iterator it = D.Stock.begin();
while (it != D.Stock.end())
{
toStream << it->first << " " << it->second.first << " " << it->second.second << " ";
}
return toStream;
}
std::istream &operator>>(std::istream &fromStream, CDealer &D)
{
// Read the name of the dealer.
fromStream >> D.m_strName_Dealer;
// Read the name of the agent.
fromStream >> D.m_strAgent;
// Read the address.
fromStream >> D.m_strAddress;
// Read the number of items.
size_t num;
fromStream >> num;
// Now read the Stock items.
for ( size_t i = 0; i < num; ++i )
{
// Obtained the types for key, val1, and val2 from
// the error message posted in a comment by OP.
CStock key;
int val1;
double val2;
fromStream >> key >> val1 >> valu2;
// Add the item to Stock.
D.Stock.insert(std::make_pair(key, std::make_pair(val1, val2)));
}
return fromStream
}

Answer 2

Problem
The problem can be reduced to the following case:
#include <map>
int main(void) {
std::map<int, int> val;
auto it = val.begin();
it->first = 10;
}
The key values in a std::map are const (the pair of key and value is defined as std::pair<const Key, T>), so for
map<CStock, pair<unsigned, double>>
we have a map containing pairs of
std::pair<const CStock, pair<unsigned, double>>
which makes it->first in
fromStream >> it->first >> it->second.first >> it->second.second;
a const CStock and results in the reported error.
This all makes sense because the map is ordered by the key, so if you could change the key whenever you wanted to whatever you wanted the map would soon be incoherent.
Documentation on std::map
Solution
There is no simple solution. You cannot do this. You must either make the items and then place them in the map or use one of the emplace family of functions to construct them directly in the map. Once they are in the map you can change the value, but not the key. You could remove the item, and thus the key, and reinsert with a different key, but why are you doing this in the first place? I recommend a rethink of your logic.