我无法编译这C++代码

我在编译此代码时遇到问题。我一直遇到令人困惑的错误，我似乎无法破解。我做错了什么？

此代码应该计算给定值的集合差、设置并集和设置交集。

#include <iostream>
#include <vector>

template<class ItemType>
int VectorBag<ItemType>::getCurrentSize() const {
    return items.size();
}
template<class ItemType>
bool VectorBag<ItemType>::isEmpty() const {
    return items.size() == 0;
}
template<class ItemType>
bool VectorBag<ItemType>::add(const ItemType& newEntry) {
    items.push_back(newEntry);
    return true;
}
template<class ItemType>
bool VectorBag<ItemType>::remove(const ItemType& anEntry) {
    for( vector<ItemType>::iterator iter = items.begin(); iter != items.end(); ++iter ) {
        if( *iter == anEntry )
        {
            items.erase( iter );
            return true;
        }
    }
    return false;
}
template<class ItemType>
void VectorBag<ItemType>::clear() {
    items.clear();
}
template<class ItemType>
bool VectorBag<ItemType>::contains(const ItemType& anEntry) {
    bool found = false;
    int i = 0;
    while (!found && (i < (int) items.size())) {
        if (anEntry == items[i])
        found = true;
        i++;
    }
    return found;
}
template<class ItemType>
int VectorBag<ItemType>::getFrequencyOf(const ItemType& anEntry) {
    int f = 0;
    for (int i = 0; i < (int) items.size(); i++) {
    if (items[i] == anEntry)
        f++;
    }
    return f;
}
template<class ItemType>
vector<ItemType> VectorBag<ItemType>::toVector() {
    vector<ItemType> vec;
    //copy all elements to new set and return
    for (int i = 0; i < (int) items.size(); i++)
        vec.push_back(items[i]);
    return vec;
}
template<class ItemType>
VectorBag<ItemType>
VectorBag<ItemType>::operator+(VectorBag<ItemType> anotherBag) {
    VectorBag<ItemType> newBag;
    //use all elements from both sets
    for (int i = 0; i < (int) items.size(); i++) {
        newBag.add(items[i]);
    }
    for (int i = 0; i < (int) anotherBag.items.size(); i++) {
        newBag.add(anotherBag.items[i]);
    }
    return newBag;
}
template<class ItemType>
VectorBag<ItemType>
VectorBag<ItemType>::operator*(VectorBag<ItemType> anotherBag) {
    VectorBag<ItemType> newBag;
    vector<ItemType> v3;
    //find intersection of sets
    sort(this->items.begin(), this->items.end());
    sort(anotherBag.items.begin(), anotherBag.items.end());
    set_intersection(this->items.begin(), this->items.end(), anotherBag.items.begin(), anotherBag.items.end(),back_inserter(v3));
    newBag.items = v3;
    return newBag;
}
template<class ItemType>
VectorBag<ItemType>
VectorBag<ItemType>::operator-(VectorBag<ItemType> anotherBag) {
    VectorBag<ItemType> newBag;
    //check if items in set1 exists in set2
    for (int i = 0; i < (int) items.size(); i++) {
        if (!anotherBag.contains(items[i])) {
            newBag.add(items[i]);
        }
    }
    return newBag;
}