编译错误 C++：无法推导出“T”的模板参数

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

提问by OneShot

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.

我正在尝试读取二进制数据以将结构加载回内存，以便我可以编辑它们并将它们保存回 .dat 文件。

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?

readVector() 尝试读取文件，并返回序列化的向量。但是当我尝试运行它时，我收到了这个编译错误。我的模板有什么问题？

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

********* 编辑 ******************

Code:

代码：

// 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:

编译器错误：

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 ??

哦，我的天……我修复了我认为的那个错误，现在我又犯了一个错误。请你也帮我解决这个问题好吗！这到底是什么意思？？

回答by dirkgently

readVector( in ); // error C2783

This should be:

这应该是：

readVector<InventoryItem>( in ); 

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

参数相关查找无助于推断模板，因此需要明确说明。

in >> tmp;

This line requires you have a function like:

此行要求您具有以下功能：

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).

放入定义（我认为 strager 在你的另一个问题中提供了这个）。

回答by Andrew Shepherd

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

当您调用 readVector 时，它通常会尝试从参数中推导出模板参数。

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

在您的示例中，唯一的模板化部分是返回值，因此无法隐式计算出来。

Change your call

改变你的电话

readVector(in)

to

到

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

回答by Naxos

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

我认为发生的事情是 ofstream 不知道如何序列化您的结构。

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

您可能需要为 InventoryItem 定义 >> 和 << 运算符：

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

Edit::

编辑：：

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

这是一个完整的示例。在编辑之前，我对 ifstream 的了解很少。

#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;
}

#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:

预期的输出是：

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

回答by kiriloff

This error occurs when compiler cannot resolve type for template argument.

当编译器无法解析模板参数的类型时会发生此错误。

Here is an example from my code:

这是我的代码中的一个示例：

template QSP<VarDataInterface<qint64>> VarDataManager::interpolateFromDB( 
const DBWrapper & _db, const QString &variablePath, const QString &_unit, 
const DBWrapper * _dbRef /*= nullptr*/, 
const QString &refPath /*= QString::null*/, 
const QString &_unitRef /*= QString::null*/ );

template QSP<VarDataInterface<QString>> VarDataManager::interpolateFromDB( 
const TA5::IDBWrapper & _db, const QString &variablePath, 
const QString &_unit, const TA5::IDBWrapper * _dbRef /*= nullptr*/, 
const QString &refPath /*= QString::null*/, const QString &_unitRef /*= QString::null*/ );

template< class TYPE >
QSP<VarDataInterface<TYPE>> VarDataManager::interpolateFromDB( 
const DBWrapper & _db,   const QString &variablePath, const QString &_unit, 
const DBWrapper * _dbRef /*= nullptr*/, 
const QString &refPath /*= QString::null*/, 
const QString &_unitRef /*= QString::null*/ )
{
    const Header dbVar = _db.getDatabaseVariable( variablePath );

    if( !dbVar.isValid() )
        return QSP<VarDataInterface<TYPE>>(nullptr);

    TVariableHeader dbVarRef;
    if( _dbRef != nullptr )
    {
        //
    }
    else
    {
        //
    }

     return QSP<VarDataInterface<TYPE>>();
}

used here:

在这里使用：

in a first attempt i wrote this

在第一次尝试中，我写了这个

m_Var = VarDataManager::interpolateFromDB(VarDataManager::getMainDB(), treeitem->text(0), QString::null, nullptr, QString::null, QString::null);

and got error C2783. solved with this

并得到错误 C2783。用这个解决了

m_Var = VarDataManager::interpolateFromDB<double>(VarDataManager::getMainDB(), treeitem->text(0), QString::null, nullptr, QString::null, QString::null);

HTH

HTH