Why am i getting an overlad error when using .pushback on vector enclosed by a vector

Question

I'm trying to create a hash table with a vector inside a vector made of a struct. v[1].push_back(value); it's giving me an error:

error C2664: 'void std::vector<node,std::allocator<node>>::push_back(_Ty &&)': cannot convert argument 1 from 'int' to 'const _Ty &'
        with
        [
            _Ty=node
        ]
note: Reason: cannot convert from 'int' to 'const _Ty'
        with
        [
            _Ty=node
        ]
note: No constructor could take the source type, or constructor overload resolution was ambiguous

Here is my code: struct node { int data;

node() {
    data = 0;
}
};

class hashmap {
public:
vector<vector<struct node>> v;
vector<struct node> n;

hashmap() {
    for (int i = 0; i < 5; i++) {
        v.push_back(n);
    }
}


void insert(int key, int value) {
    int index = Hashfunction(key);
    v[1].push_back(value);

}

int Hashfunction(int key) {
    int index = key % v.size();
    return index;
}



};

Answer 1

Always look at the full error message, modern compilers tend to be quite helpful. In this case the key information is: cannot convert from 'int' to 'const _Ty' with _Ty=node which if you swap out the type template gives cannot convert from 'int' to 'const node' . This isn't related to having nested vectors. You would see the same error with the following code:

struct node {
    int data;

    node() {
        data = 0;
    }
};

vector<struct node> n;
n.push_back(1);

The error is because the compiler has no way to convert from an int to a node . The fix is to provide a constructor which takes an int :

struct node {
    int data;

    node()
    : data(0)
    {
    }

    node(int value)
    : data(value)
    {
    }
};

Note the use of initialisers rather than assigning to the members in the constructor body, this produces more efficient code.

Ideally constructors taking a single argument should be marked explicit to help prevent issues like ambiguities:

struct node {
    int data;

    node()
    : data(0)
    {
    }

    explicit node(int value)
    : data(value)
    {
    }
};

Note you need to slightly change your push_back call to explicitly create a node :

v[1].push_back(node(value));

Or slightly more efficient and less typing:

v[1].emplace_back(value)