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?

Why are you using calloc? And any mention of "dynamic arrays" in C++ will lead to the obligatory "why are you not using std::vector"? — PaulMcKenzie
– PaulMcKenzie, Commented May 7, 2015 at 17:17

b4hand · Accepted Answer · 2015-05-07 17:20:38Z

2

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.

edited May 7, 2015 at 17:20

answered May 7, 2015 at 17:14

b4hand

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Mike Seymour · Accepted Answer · 2015-05-07 17:16:20Z

1

Why on earth are you messing around with pointers like this?

If you really want an array of pointers, then you'll have to dereference to access the map itself:

(*my_dict)[some_key]++;

assuming you've correctly set up each pointer to point to a valid dictionary.

Or use a less insane data structure

std::vector<dictionary> dict_array(input_size);

dict_array[i][some_key]++;

answered May 7, 2015 at 17:16

Mike Seymour

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pcdangio · Accepted Answer · 2020-03-16 19:20:26Z

0

use operator[]():

mydict->operator[](some_key)++;

answered Mar 16, 2020 at 19:20

pcdangio

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