CODEWITHSUNDEEP
c++
dimitris93
dimitris93

Reputation: 4273

How to create an array of vectors

How can I initialize an array of vectors properly ?

Graph.h

class Graph
{
public:
    Graph(int vertices); 
private:
    vector<int> adjacencyLists[];
}

Graph.cpp

Graph::Graph(int vertices)
{       
    adjacencyLists = new vector<int>[vertices];
}

Error:

error: incompatible types in assignment of 'std::vector<int>*' to 'std::vector<int> [0]'
 adjacencyLists = new vector<int>[vertices];
                ^

Upvotes: 0

Views: 310

Answers (2)

David
David

Reputation: 28168

The perfect thing to you for your use case (a dynamically allocated array which can't change size after construction) is std::dynarray. Sadly, this is currently not in the standard. Here's how it would work:

dynarray<vector<int>> adjacencyLists;

Graph::Graph(int vertices) : adjacencyLists{vertices} {}

Since it's not in the standard yet, I'd go with a vector (which is really a superset of dynarray, so its use is literally identical):

vector<vector<int>> adjacencyLists;

Graph::Graph(int vertices) : adjacencyLists{vertices} {}

Or if you really want to manage it all yourself, you could store it in a unique_ptr:

unique_ptr<vector<int>[]> adjacencyLists;
int numVertices;

Graph::Graph(int vertices)
    : adjacencyLists{new vector<int>[vertices]}
    , numVertices{vertices}
{}

With this approach you almost definitely want to store the size of adjacencyLists as well (as shown above), otherwise you won't be able to iterate through the array as you won't know how big it is.

No matter what you do, never have a raw pointer own memory. It should always be a unique_ptr, shared_ptr or some container. If you use the delete keyword, it's not modern c++.

Upvotes: 2

user2209008
user2209008

Reputation:

Change:

vector<int> adjacencyLists[];

To:

vector<int>* adjacencyLists;

Upvotes: 1

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