Google's style guide about input/output parameters as pointers

Question

The Google C++ Style Guide draws a clear distinction (strictly followed by cpplint.py) between input parameters(→ const ref, value) and input-output or output parameters (→ non

Answer 1

You're first question: "So, what's the point to always demand a pointer if I want to avoid the pointer to be null?"

Using a pointer announces to the caller that their variable may be modified. If I am calling foo(bar), is bar going to be modified? If I am calling foo(&bar) it's clear that the value of bar may be modified.
There are many examples of functions which take in a null indicating an optional output parameter (off the top of my head time is a good example.)

Your second question: "Why only use references for input arguments?"

Working with a reference parameter is easier than working with a pointer argument.

int foo(const int* input){
    int return = *input;

    while(*input < 100){
        return *= *input;
        (*input)++;
    }
}

This code rewritten with a reference looks like:

int foo(const int& input){
    int return = input;

    while(input < 100){
        return *= input;
        input++;
    }
}

You can see that using a const int& input simplifies the code.