Accessing the values of map from its key using pointer to map

Question

I want to dynamically allocate an array of pointers to an unordered_map in C++. The std::unordered map has been typedef as 'dictionary'.

dict_array= ( dictionary **) calloc(input_size, sizeof(dictionary*));

Now I want to access the individual hashmaps, and for each individual hashmap (mydict), I want to access the values using some key. like below:

for (int i=0; i< input_size, i++){
   dictionary *mydict= dict_array[i];
   mydict[some_key]++;  /*access the value against 'some_key' and increment it*/
}

But this above line to access the value against the key generates a compilation error. What would be the correct way to access it?

Answer 1

In your example, you haven't actually allocated any dictionary or (std::unordered_map) objects yet.

The dict_array[i] is simply a null pointer. Thus the assignment to mydict also results in a null pointer. You would need to construct a dictionary first by invoking dict_array[i] = new dictionary();.

The expression mydict[some_key]++ doesn't mean what you think it does because mydict is a dictionary * and not a dictionary. Thus you would need to actually dereference it first before having access to a valid dictionary object:

(*my_dict)[some_key]++

But again, before this would work, you need to initialize the underlying pointers.

Also, it's generally a bad idea (which often leads to undefined behavior) to mix C allocation with C++ standard objects.