Linker Error undefined symbols C++

Question

I have been getting a lot of trouble from linker errors. This is the most recent one I have gotten.

Undefined symbols for architecture x86_64:
"XMLObject::~XMLObject()", referenced from:
  Hash::addFile(char*, XMLObject) in hash.o
  std::__1::__split_buffer<XMLObject, std::__1::allocator<XMLObject>&>::~__split_buffer() in hash.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

I get it when i add a hash table (.h and .cpp). I really dont know what could cause this error.

void Hash::addObj(char* id, XMLObject num)
{
int index = hash(id);
if(hashTable[index]->identifier == "empty") // checks empty bucket
{
    hashTable[index]->identifier = id;
    hashTable[index]->files.push_back(num);
}
else // checks list
{
    bool check = false; // true if id word was found
    item* ptr = hashTable[index];
    while(ptr != nullptr)
    {
        if(ptr->identifier == id)
        {
            ptr->files.push_back(num);
            check = true;
            break;
        }
        ptr = ptr->next;
    }
    if(!check)
    {
        item* n = new item;
        n->identifier = id;
        n->files.push_back(num);
        n->next = nullptr;
        ptr = n;
    }
}
}

Answer 1

It looks like you added to your project hash.h and hash.cpp, but that module depends on another one, maybe xmlobject.h and xmlobject.cpp.

Solution: add these other files to the project too.