Compile error C++: could not deduce template argument for 'T'

Question

I'm trying to read binary data to load structs back into memory so I can edit them and save them back to the .dat file.

readVector() attempts to read the file, and return the vectors that were serialized. But i'm getting this compile error when I try and run it. What am I doing wrong with my templates?

********* EDIT ******************

Code:

// Project 5.cpp : main project file.

#include "stdafx.h"
#include <iostream>
#include <fstream>
#include <string>
#include <vector>
#include <algorithm>

using namespace System;
using namespace std;
#pragma hdrstop

int checkCommand (string line);

template<typename T>
void writeVector(ofstream &out, const vector<T> &vec);

template<typename T>
vector<T> readVector(ifstream &in);

struct InventoryItem {
    string Item;
    string Description;
    int Quantity;
    int wholesaleCost;
    int retailCost;
    int dateAdded;
} ;


int main(void)
{
    cout << "Welcome to the Inventory Manager extreme! [Version 1.0]" << endl;

    ifstream in("data.dat");

    vector<InventoryItem> structList;

    readVector<InventoryItem>( in );

    while (1)
    {

        string line = "";

        cout << endl;
        cout << "Commands: " << endl;
        cout << "1: Add a new record " << endl;
        cout << "2: Display a record " << endl;
        cout << "3: Edit a current record " << endl;
        cout << "4: Exit the program " << endl;
        cout << endl;
        cout << "Enter a command 1-4: ";

        getline(cin , line);


        int rValue = checkCommand(line);
        if (rValue == 1)
        {
            cout << "You've entered a invalid command! Try Again." << endl;
        } else if (rValue == 2){ 
            cout << "Error calling command!" << endl;
        } else if (!rValue) {
            break;
        }
    }


    system("pause");

    return 0;
}

int checkCommand (string line)
{
    int intReturn = atoi(line.c_str());
    int status = 3;

    switch (intReturn)
    {
        case 1:
            break;
        case 2:
            break;
        case 3:
            break;
        case 4:
            status = 0;
            break;
        default:
            status = 1;
            break;
    }
    return status;
}

template<typename T>
void writeVector(ofstream &out, const vector<T> &vec)
{
    out << vec.size();

    for(vector<T>::const_iterator i = vec.begin(); i != vec.end(); i++)
    {
        out << *i;
    }
}

ostream& operator<<(std::ostream &strm, const InventoryItem &i) {
  return strm << i.Item << " (" << i.Description << ")";
}


template<typename T>
vector<T> readVector(ifstream &in)
{
    size_t size;
    in >> size;

    vector<T> vec;
    vec.reserve(size);

    for(int i = 0; i < size; i++)
    {
        T tmp;
        in >> tmp;
        vec.push_back(tmp);
    }

    return vec;
}

Compiler errors:

1>------ Build started: Project: Project 5, Configuration: Debug Win32 ------
1>Compiling...
1>Project 5.cpp
1>.\Project 5.cpp(124) : warning C4018: '<' : signed/unsigned mismatch
1>        .\Project 5.cpp(40) : see reference to function template instantiation 'std::vector<_Ty> readVector<InventoryItem>(std::ifstream &)' being compiled
1>        with
1>        [
1>            _Ty=InventoryItem
1>        ]
1>.\Project 5.cpp(127) : error C2679: binary '>>' : no operator found which takes a right-hand operand of type 'InventoryItem' (or there is no acceptable conversion)
1>        C:\Program Files (x86)\Microsoft Visual Studio 9.0\VC\include\istream(1144): could be 'std::basic_istream<_Elem,_Traits> &std::operator >><std::char_traits<char>>(std::basic_istream<_Elem,_Traits> &,signed char *)'
1>        with
1>        [
1>            _Elem=char,
1>            _Traits=std::char_traits<char>
1>        ]
1>        C:\Program Files (x86)\Microsoft Visual Studio 9.0\VC\include\istream(1146): or       'std::basic_istream<_Elem,_Traits> &std::operator >><std::char_traits<char>>(std::basic_istream<_Elem,_Traits> &,signed char &)'
1>        with
1>        [
1>            _Elem=char,
1>            _Traits=std::char_traits<char>
1>        ]
1>        C:\Program Files (x86)\Microsoft Visual Studio 9.0\VC\include\istream(1148): or       'std::basic_istream<_Elem,_Traits> &std::operator >><std::char_traits<char>>(std::basic_istream<_Elem,_Traits> &,unsigned char *)'
1>        with
1>        [
1>            _Elem=char,
1>            _Traits=std::char_traits<char>
1>        ]
1>        C:\Program Files (x86)\Microsoft Visual Studio 9.0\VC\include\istream(1150): or       'std::basic_istream<_Elem,_Traits> &std::operator >><std::char_traits<char>>(std::basic_istream<_Elem,_Traits> &,unsigned char &)'
1>        with
1>        [
1>            _Elem=char,
1>            _Traits=std::char_traits<char>
1>        ]
1>        C:\Program Files (x86)\Microsoft Visual Studio 9.0\VC\include\istream(155): or       'std::basic_istream<_Elem,_Traits> &std::basic_istream<_Elem,_Traits>::operator >>(std::basic_istream<_Elem,_Traits> &(__cdecl *)(std::basic_istream<_Elem,_Traits> &))'
1>        with
1>        [
1>            _Elem=char,
1>            _Traits=std::char_traits<char>
1>        ]
1>        C:\Program Files (x86)\Microsoft Visual Studio 9.0\VC\include\istream(161): or       'std::basic_istream<_Elem,_Traits> &std::basic_istream<_Elem,_Traits>::operator >>(std::basic_ios<_Elem,_Traits> &(__cdecl *)(std::basic_ios<_Elem,_Traits> &))'
1>        with
1>        [
1>            _Elem=char,
1>            _Traits=std::char_traits<char>
1>        ]
1>        C:\Program Files (x86)\Microsoft Visual Studio 9.0\VC\include\istream(168): or       'std::basic_istream<_Elem,_Traits> &std::basic_istream<_Elem,_Traits>::operator >>(std::ios_base &(__cdecl *)(std::ios_base &))'
1>        with
1>        [
1>            _Elem=char,
1>            _Traits=std::char_traits<char>
1>        ]
1>        C:\Program Files (x86)\Microsoft Visual Studio 9.0\VC\include\istream(175): or       'std::basic_istream<_Elem,_Traits> &std::basic_istream<_Elem,_Traits>::operator >>(std::_Bool &)'
1>        with
1>        [
1>            _Elem=char,
1>            _Traits=std::char_traits<char>
1>        ]
1>        C:\Program Files (x86)\Microsoft Visual Studio 9.0\VC\include\istream(194): or       'std::basic_istream<_Elem,_Traits> &std::basic_istream<_Elem,_Traits>::operator >>(short &)'
1>        with
1>        [
1>            _Elem=char,
1>            _Traits=std::char_traits<char>
1>        ]
1>        C:\Program Files (x86)\Microsoft Visual Studio 9.0\VC\include\istream(228): or       'std::basic_istream<_Elem,_Traits> &std::basic_istream<_Elem,_Traits>::operator >>(unsigned short &)'
1>        with
1>        [
1>            _Elem=char,
1>            _Traits=std::char_traits<char>
1>        ]
1>        C:\Program Files (x86)\Microsoft Visual Studio 9.0\VC\include\istream(247): or       'std::basic_istream<_Elem,_Traits> &std::basic_istream<_Elem,_Traits>::operator >>(int &)'
1>        with
1>        [
1>            _Elem=char,
1>            _Traits=std::char_traits<char>
1>        ]
1>        C:\Program Files (x86)\Microsoft Visual Studio 9.0\VC\include\istream(273): or       'std::basic_istream<_Elem,_Traits> &std::basic_istream<_Elem,_Traits>::operator >>(unsigned int &)'
1>        with
1>        [
1>            _Elem=char,
1>            _Traits=std::char_traits<char>
1>        ]
1>        C:\Program Files (x86)\Microsoft Visual Studio 9.0\VC\include\istream(291): or       'std::basic_istream<_Elem,_Traits> &std::basic_istream<_Elem,_Traits>::operator >>(long &)'
1>        with
1>        [
1>            _Elem=char,
1>            _Traits=std::char_traits<char>
1>        ]
1>        C:\Program Files (x86)\Microsoft Visual Studio 9.0\VC\include\istream(309): or       'std::basic_istream<_Elem,_Traits> &std::basic_istream<_Elem,_Traits>::operator >>(__w64 unsigned long &)'
1>        with
1>        [
1>            _Elem=char,
1>            _Traits=std::char_traits<char>
1>        ]
1>        C:\Program Files (x86)\Microsoft Visual Studio 9.0\VC\include\istream(329): or       'std::basic_istream<_Elem,_Traits> &std::basic_istream<_Elem,_Traits>::operator >>(__int64 &)'
1>        with
1>        [
1>            _Elem=char,
1>            _Traits=std::char_traits<char>
1>        ]
1>        C:\Program Files (x86)\Microsoft Visual Studio 9.0\VC\include\istream(348): or       'std::basic_istream<_Elem,_Traits> &std::basic_istream<_Elem,_Traits>::operator >>(unsigned __int64 &)'
1>        with
1>        [
1>            _Elem=char,
1>            _Traits=std::char_traits<char>
1>        ]
1>        C:\Program Files (x86)\Microsoft Visual Studio 9.0\VC\include\istream(367): or       'std::basic_istream<_Elem,_Traits> &std::basic_istream<_Elem,_Traits>::operator >>(float &)'
1>        with
1>        [
1>            _Elem=char,
1>            _Traits=std::char_traits<char>
1>        ]
1>        C:\Program Files (x86)\Microsoft Visual Studio 9.0\VC\include\istream(386): or       'std::basic_istream<_Elem,_Traits> &std::basic_istream<_Elem,_Traits>::operator >>(double &)'
1>        with
1>        [
1>            _Elem=char,
1>            _Traits=std::char_traits<char>
1>        ]
1>        C:\Program Files (x86)\Microsoft Visual Studio 9.0\VC\include\istream(404): or       'std::basic_istream<_Elem,_Traits> &std::basic_istream<_Elem,_Traits>::operator >>(long double &)'
1>        with
1>        [
1>            _Elem=char,
1>            _Traits=std::char_traits<char>
1>        ]
1>        C:\Program Files (x86)\Microsoft Visual Studio 9.0\VC\include\istream(422): or       'std::basic_istream<_Elem,_Traits> &std::basic_istream<_Elem,_Traits>::operator >>(void *&)'
1>        with
1>        [
1>            _Elem=char,
1>            _Traits=std::char_traits<char>
1>        ]
1>        C:\Program Files (x86)\Microsoft Visual Studio 9.0\VC\include\istream(441): or       'std::basic_istream<_Elem,_Traits> &std::basic_istream<_Elem,_Traits>::operator >>(std::basic_streambuf<_Elem,_Traits> *)'
1>        with
1>        [
1>            _Elem=char,
1>            _Traits=std::char_traits<char>
1>        ]
1>        while trying to match the argument list '(std::ifstream, InventoryItem)'
1>Build log was saved at "file://c:\Users\Owner\Documents\Visual Studio 2008\Projects\Project 5\Project 5\Debug\BuildLog.htm"
1>Project 5 - 1 error(s), 1 warning(s)
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

Oh my god...I fixed that error I think and now I got another one. Will you PLEASE just help me on this one too! What the heck does this mean ??

Solution 1

readVector( in ); // error C2783

This should be:

readVector<InventoryItem>( in ); 

Argument dependent lookup does not help deduce the templates, hence explicit specification is required.

in >> tmp;

This line requires you have a function like:

istream& operator>>(ostream& is, InventoryItem& item) {
    // parse and read in the data to item
    return is;
}

Put in the definition (I think strager provided this in your other question).

Solution 2

When you call readVector, it would normally try to deduce the template arguments from the parameter.

In your example, the only templated part is the return value, so it has no way to figure it out implicitly.

Change your call

readVector(in)

to

vector<InventoryItem> structList = readVector<InventoryItem>(in);

Solution 3

I think what's happening is that ofstream does not know how to serialize your struct.

You may need to define >> and << operators for InventoryItem:

ofstream& operator << ( ofstream& ofs, InventoryItem& );
ifstream& operator >> ( ifstream& ifs, InventoryItem& );

Edit::

Here's a fully worked example. I was little off on the ifstream before the edit.

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

struct InventoryItem{
    std::string name;
    int id;
    float price;
};

using std::ofstream;
using std::ifstream;

ofstream& operator << (ofstream& ofs, InventoryItem& ii)
{
    ofs << ii.name << std::endl 
        << ii.id << std::endl
        << ii.price << std::endl;
    return ofs;
}

ifstream& operator >> ( ifstream& ifs, InventoryItem& ii )
{
    ifs >> ii.name >> ii.id >> ii.price;
    return ifs;
}

void printItem(const InventoryItem& ii)
{
    std::cout << "Name: " << ii.name << std::endl
        << "ID: " << ii.id << std::endl
        << "Price: " << ii.price << std::endl;
}

int main()
{
    ofstream outfile("testout.txt");
    InventoryItem list[] = { {"Item1", 1, 1.5}, {"Item2", 2, 2.5} };

    outfile << list[0] << list[1];
    outfile.close();

    std::ifstream infile("testout.txt");
    InventoryItem list2[2]; 

    infile >> list[0] >> list[1];

    printItem(list[0]);
    printItem(list[1]);

    return 0;
}

The expected output is:

    Name: Item1
    ID: 1
    Price: 1.5
    Name: Item2
    ID: 2
    Price: 2.5