C++ how to fix this error "Allocating an object of abstract class type"

void check (bool success) {if (success) cout NestedLessLess "Done" NestedLessLess end1; else cout NestedLessLess "Entry not in tree." NestedLessLess end1;}//end check int main() {Binary SearchTree* tree1Ptr = new Binary SearchTree(); tree1Ptr rightarrow add("10"); tree1Ptr rightarrow add("20"); tree1Ptr rightarrow add("30"); tree1Ptr rightarrow add("40"); tree1Ptr rightarrow add("50"); tree1Ptr rightarrow add("60"); tree1Ptr rightarrow add("70"); tree1Ptr rightarrow add("80"); cout NestedLessLess "Tree 1 Preorder: Should be 10 20 30 40 50 60 70 80 '; tree1Ptr rightarrow preorderTraverse(display)