How to initialize unordered_map<string, unordered_set<string>> members using initializer list?

Question

I have this class

class A {
    unordered_map<string, unordered_set<string>> n_;
  public:
    A(unordered_map<string, unordered_set<string>>& n) : n_{n} {}
};

And I want to be able to use the constructor with that syntax

int main() {
    A a{{"C", {"A", "B"}}};
    return 0;
}

But in the way it's written now, I'm getting error

error: no matching function for call to `‘A::A(<brace-enclosed initializer list>)’ A a{{"C", {"A", "B"}}};`

How can it be fixed?

Answer 1

You need to add one more {} for it. And note that temporary can't be bound to lvalue-reference to non-const. (They could be bound to lvalue-references to const or rvalue-references.) e.g.

class A {
    unordered_map<string, unordered_set<string>> n_;
  public:
    A(const unordered_map<string, unordered_set<string>>& n) : n_{n} {}
    //^^^^^
};

int main() {
    A a{{{"C", {"A", "B"}}}};
    //          ^^^  ^^^     elements of unordered_set
    //         ^^^^^^^^^^    for the unordered_set
    //   ^^^^^^^^^^^^^^^^^   elements (std::pair) of unordered_map (only one here)
    //  ^^^^^^^^^^^^^^^^^^^  for the unordered_map
    // ^^^^^^^^^^^^^^^^^^^^^ for A

    return 0;
}

I guess you might miss the {} for the elements (std::pair) of the unordered_map; in a similar fashion, if you want to make the unordered_map containing two elements, you can write it as

A b{{{"C", {"A", "B"}}, {"F", {"D", "E"}}}};

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