Reference to STL Vector Type
===

I am using someone elses function that takes: 
>std::vector<double> &input

How should I pass a vector to this, I have tried:
    
    std::vector<double> input;
    input.push_back('several points');
    optimise(input);
This has a linker error saying:

    undefined reference to optimise(std::vector<double, std::allocator<double> >&)

If i try this:

     std::vector<double> &input;
    input.push_back('several points');
    optimise(input);

Then there is a compile error:

    'input' declared as reference but not initialised

How do I initialise a reference to the vector or am I doing something completely wrong. 