How do I reference a vector from a map's value?-CodePudding

#include <iostream>
#include <vector>
#include <map>
using namespace std;

int main() {
  vector<string> examplevector {"one", "two", "three"};
  map<string, vector<string>> examplemap {{"vector1", examplevector}};

  examplemap["vector1"][0] = "eight";

  cout << examplemap["vector1"][0] << endl; // prints "eight"
  cout << examplevector[0] << endl; // prints "one". I was expecting "eight".

}

Is there a way to alter the values within examplevector via examplemap["vector1"]?

CodePudding user response：

No, C does not work this way, this is not how objects work in C . This is how objects work in Java and C#, but C is not Java or C#. Objects that are stored in some container, a vector, a map, or any other container, are distinct objects of their own and have nothing to do with any other object.

map<string, vector<string>> examplemap {{"vector1", examplevector}};

This effectively makes a copy of the examplevector and stores it in the map. Changes to the object in the map have no effect on the original examplevector. Similarly:

string abra="cadabra";

std::vector<std::string> examplevector{"hocuspocus", abra};

// ...
examplevector[1]="abracadabra";

This modifies the object in the vector. abra is still cadabra.

It's possible to store a map or a container of pointers, or perhaps std::reference_wrappers. Then, modify the pointed-to object, or wrapped reference, will modify the underlying object. But that introduces its own kind of complexity, in terms of managing the lifetime of all objects, correctly.

CodePudding user response：

If you want to alter the values of examplevector via your map you should declare your map as a map of strings associated to vector pointers, therefore you can alter the original vector using the its reference in the map.

Adapting your code, it will become like:

#include <iostream>
#include <vector>
#include <map>
using namespace std;

int main() {
  vector<string>* examplevector = new vector<string>({"one", "two", "three"});
  map<string, vector<string>*> examplemap = {{"vector1", examplevector}};

  examplemap["vector1"]->at(0) = "eight";

  cout << examplemap["vector1"]->at(0) << endl; // prints "eight"
  cout << examplevector->at(0) << endl; // prints "eigth", as expected.

}