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

Question

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?

Thanks. I forgot it. I knew that I am missing something obvious. — Hakim
– Hakim, Commented Nov 23, 2015 at 16:32

LogicStuff · Accepted Answer · 2015-11-23 16:40:44Z

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: ?).

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