C++ :: Accessing Data Declared Private Within Header File?

I have a header file that declares some fields as private, I then have a class that I need to compare two of the objects' information for equality but neither of them are the calling objects. I cannot alter the header file. How would I go about comparing private data fields? I will enter a brief bit of code for clarity.

Code: // Header File
// stuff.h

class stuff
{
private:
int* arr[20];
int size;
};
bool equal (const stuff& a, const stuff& b);

[code].....