Simple passing of Matrices ie. cv::Mat to functions in OpenCV2.4

I am new to opencv and I have a doubt with one of the simplest operations in coding: Passing values to functions.

Heres what I want to do,

1. Initialize a cv:Mat in the main function with say, values from 0 to 50.
2. Pass this matrix as an argument to a function foo() which in turn simply prints out the values of each element in the matrix. Thats it.

Now I have read in multiple places that when these matrices are passed, it is only the header that gets passed, not the actual data. Then how am I still able to read out the values of this matrix in the called function as if the entire matrix was passed?