CODEWITHSUNDEEP
c++templatespointers
pedr0
pedr0

Reputation: 3011

C++ double pointer not known conversion

I am literally freaking out on the following :

template <class T> // int, float, double etc..
class Graph {
public:

  // Documentation, it has to be between [0..100]
  Graph(int size = 10, int density = 10, T range = 0):
    m_size(size),
    m_density(density),
    m_range(range) {
    generate();
  }

  ~Graph() {
     for (int i = 0; i < m_size; i++)
    delete[] m_graph[i];
      delete[] m_graph;
  }

  [..]

  static Graph<T>* custom(T** cgraph, int size, T range) {
    Graph<T> *graph = new Graph<T>(size, 10, range);
    for (int i = 0; i < size; i++)
      delete[] graph->m_graph[i];
    delete[] graph->m_graph;
    graph->m_graph = cgraph;
  }


private:
  T** m_graph;
  [ .. ]
};

int nodes[4][4] = {
  { 6, 5, 2, 5 },
  { 5, 6, 3, 3 },
  { 1, 3, 6, 1 },
  { 5, 3, 1, 6 }
};

int main() {
    Graph<int> *graph = Graph<int>::custom(nodes, 4, 5);
}

What is it poorly failing to compile reporting the following errors ?

   g++ graph.cpp -o test_graph 
graph.cpp: In function ‘int main()’:
graph.cpp:191:55: error: no matching function for call to ‘Graph<int>::custom(int [4][4], int, int)’
     Graph<int> *graph = Graph<int>::custom(nodes, 4, 5);
                                                       ^
graph.cpp:60:20: note: candidate: static Graph<T>* Graph<T>::custom(T**, int, T) [with T = int]
   static Graph<T>* custom(T** cgraph, int size, T range) {
                    ^
graph.cpp:60:20: note:   no known conversion for argument 1 from ‘int [4][4]’ to ‘int**’

It looks so right to me, what's wrong ?

Upvotes: 0

Views: 197

Answers (1)

Martin Bonner supports Monica
Martin Bonner supports Monica

Reputation: 28987

You need to make nodes by an array of pointers to int.

int nodes_v[4][4] = {
  { 6, 5, 2, 5 },
  { 5, 6, 3, 3 },
  { 1, 3, 6, 1 },
  { 5, 3, 1, 6 }
};

int *nodes[4] = { nodes_v[0], nodes_v[1], nodes_v[2], nodes_v[3] };

You also need to add an additional member to the Graph variable to mark that it is a custom graph, and if set, the destructor should not delete the memory.

Best to give Graph a private constructor which passes in the custom flag and doesn't bother allocating the memory if set.

  Graph(int size, int density, T range, bool custom):
    m_size(size),
    m_density(density),
    m_range(range),
    m_custom {
    }

  Graph(int size = 10, int density = 10, T range = 0):
    Graph(size, density, range, false) {
    generate();
    }

  static Graph<T>* custom(T** cgraph, int size, T range) {
        Graph<T> *graph = new Graph<T>(size, 10, range, true);
        graph->m_graph = cgraph;
  }

Finally, you need to handle copy constructor and assignment operator (start by just deleting them).

Upvotes: 1

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