Undefined Reference to function compile error C++

I'm getting the following error upon compiling:

In function `main':
/home/linux/blah/programname.cpp: 
35: undefined reference to 
`Something::foo(std::vector<int, std::allocator<int> > const&)'

Here is where the error is happening in 'main':

Something * thing = new Something();
vector<int> * vec = new vector<int>(256,0);
bar(argv[1],vec);
thing->foo(*vec);

Here is 'foo' as defined in 'Something.hpp' and 'Something.cpp':

void foo(const vector<int>& vec) {blah blah}

What is going on? I'm lost.