C++ 错误 C2679：二进制'['：未找到采用类型 'SalesItem' 的右侧操作数的运算符(或者没有可接受的转换)

我是这个网站的新手（如果我做错了，很抱歉），对c++来说也相当陌生。我在为我的c++类分配任务时遇到了一个令人讨厌的错误

c++错误c2679：二进制"["：找不到接受类型为"SalesItem"的右侧操作数的运算符（或者没有可接受的转换）

我真的不确定我需要包括什么来帮助得到答案。我已经包含了我认为最相关的类，省略了main（）的类。我还没有包括标题，但如果需要的话，我应该可以添加这些。。。

这是我发现这个错误的Invoice类，我用astriks（*）标记了错误发生的行：

#include "stdafx.h"
#include <iostream>
#include "Invoice.h"
#include <vector>
#include <string>
using namespace std;
static int nextInvoiceNum = 1000;
Invoice::Invoice(const string name, const int vectorSize)
{
    customerName = name;
    salesItems.reserve(vectorSize);
    invoiceNumber = nextInvoiceNum++;
}
void Invoice::setCustomerName(string name)
{
    customerName = name;
}
string Invoice::getCustomerName()
{
    return customerName;
}
int Invoice::getInvoiceNumber()
{
    return invoiceNumber;
}
void Invoice::display()
{
    cout << "-------------------------n";
    cout << "n Invoice #: " << getInvoiceNumber() << "n";
    cout << "n Customer name: " << getCustomerName() << "n";
    cout << "-------------------------n";
    cout << "Items Purchased";
    cout << "-------------------------n";
    for (auto &item : salesItems)
    {
********cout << salesItems[item].display() << "n";
        cout << "-------------------------n";
    }
}
void Invoice::addSalesItem(SalesItem&)
{
    salesItems.push_back(SalesItem());
}
double Invoice::getInvoiceAmount()
{
    double runningTot = 0;
    double total = 0;
    for (auto &item : salesItems)
    {
********runningTot = salesItems[item].getPartQuantity() * salesItems[item].getPartPrice();
        total += runningTot;
    }
    return total;
}
Invoice::~Invoice(void)
{
}

我试图做的是使用对象向量，当我尝试使用向量从SalesItems类调用display（）方法时，第一次出现错误。

这是SalesItem类：

#include "stdafx.h"
#include "SalesItem.h"
#include <iostream>
using namespace std;

SalesItem::SalesItem(string partNumber, string partDescription, int partQuantity, double partPrice)
{
    partNumber = "";
    partDescription = "";
    partQuantity = 0;
    partPrice = 0;
}

SalesItem::~SalesItem(void)
{
}
void SalesItem::setPartNumber(string number)
{
    partNumber = number;
}
void SalesItem::setPartDescription(string description)
{
    partDescription = description;
}
void SalesItem::setPartQuantity(int quantity)
{
    partQuantity = quantity;
}
void SalesItem::setPartPrice(double price)
{
    partPrice = price;
}
string SalesItem::getPartNumber()
{
    return partNumber;
}
string SalesItem::getPartDescription()
{
    return partDescription;
}
int SalesItem::getPartQuantity()
{
    return partQuantity;
}
double SalesItem::getPartPrice()
{
    return partPrice;
}
void SalesItem::display()
{
    cout<< "-------------------------:";
    cout<< "n Part number: " << getPartNumber() << "n";
    cout<< "n Part description: " << getPartDescription() << "n";
    cout<< "n Part quantity: " << getPartQuantity() << "n";
    cout<< "n Part price: $" << getPartPrice() << "n";
    cout<< "n Invoice amount: $" << getInvoiceAmount() << "n";
}
double SalesItem::operator+(double total)
{
    double subtotal = getPartQuantity() * getPartPrice();
    total += subtotal;
    return total;
}
double SalesItem::getInvoiceAmount()
{
    double invoice = getPartQuantity() * getPartPrice();
    return invoice;
}

再次，如果我以错误的方式发布了这篇文章，我很抱歉。我真的很感谢你的帮助。将及时提供任何其他必要的信息或做出更改。

谢谢！