Inherit constructor

Question

I have a class named myGraph which is derived from graph

If I know the number of vertices when I call the constructor, I will be able to use the following constructor.

myGraph(int numOfV) : graph(numOfV)
{
    // initialize...
}

I want to compute the number of vertices in the constructor and then inherit graph inside the constructor. How to do so?

myGraph(int a, int b)
{
    /* using a,b to initialize...  
       a sequence of push operation on (vector<T>)verticeList */

    int numOfV = this->verticeList.size();
    // inherit...
    myGraph(numOfV);  // it will not work


}

Please note that

the procedure I use to compute the number of vertices is complicated.(many lines of code rather than a+b)
the number of vertices depends on the instance variables in myGraph, so I need to initialize the members of myGraph first and then inherit the remaining graph part.

Luchian Grigore · Accepted Answer · 2013-06-06 03:19:47Z

3

If there's a lot of logic going on pre-initialization, you can separate that in a method:

struct myGraph
{
    myGraph(int a, int b) : graph(myGraph::computeNoVertices(a,b))
    {
    }
    static int computeNoVertices(int a, int b)
    {
        //whole chunk of code 
    }
};

answered Jun 6, 2013 at 3:19

Luchian Grigore

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1 Comment

aschepler Over a year ago

This is usually the answer to this type of problem. If you additionally need to use other members as lvalues during initialization, you can move those members to a private base struct.

taocp · Accepted Answer · 2013-06-06 03:12:33Z

2

You have to call base class constructor at derived class's member initialization list:

 myGraph(int a, int b) :graph(a +b)
 {                     //^^It is OK to do computation when you pass parameters to
                       //base class constructor
    // do something...
 }

answered Jun 6, 2013 at 3:12

taocp

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Comments

Vaughn Cato · Accepted Answer · 2013-06-06 03:17:10Z

1

With C++11, you can do a limited amount of this:

myGraph(int a, int b)
  : myGraph(a+b)
{
}

If your calculations are much more sophisticated, you'll want to break them out into a separate function though:

myGraph(int a,int b)
  : myGraph(calculateNumOfV(a,b))
{
}

answered Jun 6, 2013 at 3:17

Vaughn Cato

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3 Comments

taocp Over a year ago

call another constructor of the same class? delegate?

Praetorian Over a year ago

He wants to call the graph constructor, not delegate to another myGraph constructor.

Vaughn Cato Over a year ago

@Praetorian: maybe, but his example makes it look like he's wanting to call the other constructor.

Arne Mertz · Accepted Answer · 2013-06-06 06:32:08Z

the number of vertices depends on the instance variables in myGraph, so I need to initialize the members of myGraph first and then inherit the remaining graph part.

That is not possible. Base classes are always initialized before data members. However, during construction the values you assign to your members depend solely on the constructor arguments, so the base class' constructor parameters depend only on those arguments as well.

Let's say you have members m1 and m2 and want to initialize the base class with values dependent on those two members, say f(m1,m2) and g(m1,m2). Your constructor has the parameters a, b, c and you initialize m1 and m2 dependent on those ctor arguments, with x(a,b,c) and y(a,b,c). Then the ctor should look like this:

class Foo : Bar {
  int m1;
  int m2;
  static int x(int a, int b, int c);
  static int y(int a, int b, int c);
public:
  Foo(int a, int b, int c) 
    : Bar( f(x(a,b,c), y(a,b,c)), g(x(a,b,c), y(a,b,c)) )
    , m1(x(a,b,c))
    , m2(y(a,b,c)) 
  {}
};

Of course, to improve readability, you can always create more static functions to calculate the initializer for the base object:

class Foo : Bar {
  /* ... */
  static Bar calcBar(int a, int b, int c) {
    int i1 = x(a,b,c);
    int i2 = y(a,b,c);
    return Bar(f(i1,i2), g(i1,i2));
  }

public:
  Foo(int a, int b, int c) 
    : Bar( calcBar(a,b,c) )
    , m1(x(a,b,c))
    , m2(y(a,b,c)) 
  {}
};

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