C++ passing a nested unordered_map by reference and manipulating it

I have 2 classes (firstClass and secondClass) of which firstClass is a friend of secondClass, and has a private nested std::unordered_map, which I want to access it in a function of secondClass.

So basically the code is like this:

class secondClass;
typedef unordered_map STable<unsigned, unordered_map<unsigned, double> > NESTED_MAP;

class firstClass {
    friend class secondClass;

    void myfunc1(secondClass* sc) {
        sc->myfunc2(&STable);
    }

private:
    NESTED_MAP STable;
};

class secondClass {
public:
    void myfunc2(NESTED_MAP* st) {
        //Here I want to insert some elements in STable.
        //Something like:
        st[1][2] = 0.5;
    }
};

int main() {
    firstClass  fco;
    secondClass sco;
    fco.myfunc1(&sco);
    return 0;
}

The point is that if instead of the nested map, I use a simple std::unordered_map, I can easily modify it (add new elements, or change the values of some keys). But, in the nested map I can not do anything.

I know that it should be trivial, but I don't know how to solve it. Any idea?

You are not passing by reference, you're passing a pointer. This is a version using reference:

void myfunc2(NESTED_MAP &st)
{
    st[1][2] = 0.5; // valid, st dereferenced implicitly
}

And call it without address-of operator:

sc->myfunc2(STable);

If you really want to pass a pointer, it needs explicit dereferencing:

void myfunc2(NESTED_MAP *st)
{
    (*st)[1][2] = 0.5;

    // or, if you like:
    st->operator[](1)[2] = 0.5;
}

In the code you posted, you firstClass has only private members (forgot public: ?).