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Anything with a leading underscore is a reserved name in C++ (and in C). See this question for details.

Modern C++ uses nullptr rather than NULL. See this answer for why and how it's useful.

Anything with a leading underscore is a reserved name in C++ (and in C). See this question for details.

Modern C++ uses nullptr rather than NULL. See this answer for why and how it's useful.

void deleteDuplicateEntries() {
    for (ListNode<Type> *curr = _head; curr && curr->next; curr = curr->next) {
        ListNode<Type> *n = curr;
        for (  ; n && n->next; n = n->next) {
            while (n->next && curr->value == n->next->value) {
                // delete matching node
                auto victim = n->next;
                n->next = victim->next;
                --_size;
                delete victim;
            }
        }
        // does last node match?
        if (n && n->value == curr->value) {
            delete n;
            --_size;
            curr->next = nullptr;
            _tail = curr;
        }
    }
}

void deleteDuplicateEntries() {
    for (ListNode<Type> *curr = _head; curr && curr->next; curr = curr->next) {
        ListNode<Type> *n = curr;
        for (  ; n && n->next; n = n->next) {
            while (n->next && curr->value == n->next->value) {
                // delete matching node
                auto victim = n->next;
                n->next = victim->next;
                --_size;
                if (_tail == victim) {
                    _tail = n;
                }
                delete victim;
            }
        }
        // does last node match?
        if (n && n->value == curr->value) {
            delete n;
            --_size;
            curr->next = nullptr;
            _tail = curr;
        }
    }
}

Use nullptr rather than NULL

Use a more efficient algorithm

The deleteDuplicateEntries() routine is not very efficient. There's no need to traverse the entire list for each value. If we proceed from the head to the tail, we know that there cannot be any duplicates in the part of the list we've already processed, so all that's necessary is to search the remainder of the list. Here's how I'd do it:

void deleteDuplicateEntries() {
    for (ListNode<Type> *curr = _head; curr && curr->next; curr = curr->next) {
        ListNode<Type> *n = curr;
        for (  ; n && n->next; n = n->next) {
            while (n->next && curr->value == n->next->value) {
                // delete matching node
                auto victim = n->next;
                n->next = victim->next;
                --_size;
                delete victim;
            }
        }
        // does last node match?
        if (n && n->value == curr->value) {
            delete n;
            --_size;
            curr->next = nullptr;
            _tail = curr;
        }
    }
}

Use nullptr rather than NULL