I don't know how to make this function

Original question

I am writing a function called Evaluate in C++. It accepts three arrays as parameters:

double Evaluate (double inputs[], double hidden[], double outputs[]) {
    // To-Do...
}

The problem appears in this scenario:

The programmer decides to initialize the function Evaluate with only two parameters: inputs[] and outputs.

So, I was thinking of creating Evaluate like this:

double Evaluate (double inputs[], double hidden[] = {}, double outputs[]) {
    // To-Do...
}

But, this creates strange Errors:

  In function 'double Evaluate (double*, double*, double*)'
34:53: error: unexpected '{' token
34:54: error: unexpected '}' token

Is there a solution?

*Thanks in advance.

Updated question

I have managed to use my answer with the help in the comments.

I am currently curious, won't multiple function overloads cause the program to get slower?

Solution

One way which I have learnt is function overloading - where you create copy of the same function, but in different ways.

int add(int a)
{
    return ++a;
}

int add(int a, int b)
{
    return a + b;
}

double add(double a, double b)
{
    return a + b;
}

This became so helpful that I'm able to implement many operations with it!

In terms of my evaluate function, I could do:

evaluate(std::vector<double> inputs, std::vector<double> outputs, std::vector<double> hidden)
{
    // ...
}

evaluate (std::vector<double> inputs, std::vector<double> outputs)
{
    // ...
}