CODEWITHSUNDEEP
c++pointers
user707549
user707549

Reputation:

Pointer to a map

If I define a pointer-to-map like this:

map<int, string>* mappings;

mappings is a pointer. How should I use this pointer to operate the map?

Upvotes: 21

Views: 40627

Answers (6)

psykid
psykid

Reputation: 448

With the introduction of "at" function in c++ 11, you can use mappings->at(key) instead of (*mapping)[key].

Keep in mind that this api will throw out_of_range exception if the key is not already available in the map.

Upvotes: 2

Aman Agarwal
Aman Agarwal

Reputation: 755

Well, STL is designed to reduce the complexity of pointer handling..so better approach is to use stl::iterator.. try to avoid pointers :-/

Upvotes: -3

Nicola Musatti
Nicola Musatti

Reputation: 18218

For instance:

#include <map>
#include <string>
#include <iostream>

int main() {
  std::map<int, std::string> * mapping = new std::map<int, std::string>();
  (*mapping)[1] = "test";
  std::cout << (*mapping)[1] <<std::endl;
}

Upvotes: 3

iammilind
iammilind

Reputation: 69988

Not much difference except that you have to use -> for accessing the map members. i.e.

mapping->begin() or mapping->end()

If you don't feel comfortable with that then you can assign a reference to that and use it as in the natural way:

map<int, string> &myMap = *mappings;  // 'myMap' works as an alias
                ^^^^^^^^

Use myMap as you generally use it. i.e.

myMap[2] = "2";
myMap.begin() or myMap.end();

Upvotes: 5

ofir
ofir

Reputation: 58

another nice way of using pointers, which I like is calling mappings->(and the function you want to call)

Upvotes: 0

Lightness Races in Orbit
Lightness Races in Orbit

Reputation: 385106

Use the pointer just like you use any other pointer: dereference it to get to the object to which it points.

typedef std::map<int, string>::iterator it_t;

it_t   it1 = mappings->begin();   // (1)
it_t   it2 = (*mappings).begin(); // (2)

string str = (*mappings)[0];      // (3)

Remember that a->b is — mostly — equivalent to (*a).b, then have fun!

(Though this equivalence doesn't hold for access-by-index like (*a)[b], for which you may not use the -> syntax.)

Upvotes: 24

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