Is a member initializer list part of the declaration or the definition of a constructor?

Please explain how to use member initializer lists. I have a class declared in a .h file and a .cpp file like this:

class Example
{
private:
    int m_top;
    const int m_size;
    /* ... */
public:
    Example(int size, int grow_by = 1) : m_size(5), m_top(-1);
    /* ... */
    ~Example();
};

I'm initializing m_size on object creation because of const. How should I write the constructor? Should I repeat : m_size(5), m_top(-1), or I can omit this step?

Example::Example(int size, int grow_by)
{
    /* ... */
}

or

Example::Example(int size, int grow_by) : m_size(5), m_top(-1)
{
    /* ... some code here */
}

This is the initializer list:

Example::Example( int size, int grow_by) : m_size(5), m_top(-1)
{
    /* ... */
}

and it should be done only in the .cpp file.

If you did it in the header like in your example, you would get an error.