Trouble creating a instance of a class within another class

Question

I've got this class called Heap. I'm trying to have an instance of Heap as a private member in a class called Graph but that doesn't seem to work.

class Heap{
  private:
    Node* container;
    int size; 
    //... some more attrs ...
  public: 
    Heap(int inSize){
      size = inSize
      //... initialize other private attrs ...
    }

class Graph {
   private: 
     int size; 
     Heap h(90); 
   public: 
     Graph(int inSize){
       size = inSize;
     }

After looking at this post, How to create an object inside another class with a constructor?, I still can't figure out why I am not allow to initialize Heap within the class Graph. My guess is that private members can't be initialized, they're just placeholders so no physical memory is given to them. One of the comments in this post states that having a pointer is not a good practice.

So my questions are: 1. why isn't this a good practice? if it indeed isn't a good practice. 2. why can't I instantiate another class object from within another class's private attributes. 3. is there another way besides a pointer to solve this problem? 4. Also, it seems that if I don't have a custom ctor, and just use a default one, it works, again, I have no idea why.

Use member initializer lists in your constructor. – πάντα ῥεῖ Nov 20 '15 at 22:45 — πάντα ῥεῖ, Nov 20 '15 at 22:45
can you elaborate as to why this will not work? – jason adams Nov 20 '15 at 22:46 — jason adams, Nov 20 '15 at 22:46
Because `Heap h(90);` is simply wrong syntax. – πάντα ῥεῖ Nov 20 '15 at 22:47 — πάντα ῥεῖ, Nov 20 '15 at 22:47

score 0 · Answer 1 · answered Nov 20 '15 at 22:49

I'm trying to have an instance of Heap as a private member in a class called Graph but that doesn't seem to work

You are using the wrong syntax.

Heap h(90);

is not a valid syntax. Use:

Heap h;

Initialize it the constructor using

 Graph(int inSize) : size(inSize), h(90) { } // Or the appropriate size.

Trouble creating a instance of a class within another class

1 Answers1