CODEWITHSUNDEEP
c++
Void
Void

Reputation: 153

Instantiating an object as class member without default constructor

I am a newbie to C++. I want to declare an object as a private member of another class. Is the instantiation of this object can be done with out the default constructor? A sample code is listed here:

class Vertex{
  int i;
public: 
  Vertex(int j):i(j){};
  Vertex(const Vertex & v){i=v.i;};
}

class Test{
  Vertex v;
public:
  Test(const Vertex & v1){v(v1);}
}

int main()
{//some code here;
  Vertex v1(int j=1);
  Test(v1); // *error: no matching function for call to 'Vertex::Vertex()'*
  return 0;
}

It appears to me once declared an object as a private class member (e.g. Vertex v), the default constructor is immediately sought after. Anyway to avoid this? Thanks a lot.

Upvotes: 4

Views: 5634

Answers (3)

Konrad Rudolph
Konrad Rudolph

Reputation: 546073

Vertex v1(int j=1);

This declares a function, v1, which returns a Vertex and takes an int argument. What you want is

Vertex v1(1);

in addition, you need to use initialiser lists as the other answers have shown:

Test(const Vertex & v1) : v(v1) {}

Upvotes: 4

juanchopanza
juanchopanza

Reputation: 227578

Use the constructor initialization list:

Test(const Vertex & v1) : v(v1) {}

Once you're in the constructor's body (between the { and }), all data members have been initialized. If you don't do it explicitly, they get default initialized, which requires an available default constructor for user defined types.

Next, you are declaring a function here:

Vertex v1(int j=1);

What you probably meant is 

Vertex v1(1);

Upvotes: 1

Lightness Races in Orbit
Lightness Races in Orbit

Reputation: 385385

Initialise Test::v just as you initialised Vertex::i. (It being private is entirely irrelevant!)

So, using Test::Test's member initialisation list:

Test(const Vertex & v1) : v(v1) {};

For consistency, then, I would also suggest:

Vertex(const Vertex & v) : i(v.i) {};

Initialise members wherever you can.

Upvotes: 2

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