Ctor takes 1 arg yet able to declare an instance without args

[SSteven]

Hi

I have a very simple class named Vector, which can store a specified number of doubles. (This example is from Stroustrup's book).

The class is a handle to an array of doubles. It contains a pointer to the array:

class Vector
{
private:
  double* elem;			// pointer to the elements
  int sz;			// the number of elements

public:
  // Constructor: Acquire resources.
  Vector(int s)	
  {
    elem = new double[s];
    sz = s;

    // Initialize elements.
    for (int i = 0; i < sz; ++i)
      elem[i] = 0;
  }

  /// ...
};		/// class 

Note that the ctor takes 1 arg.

As expected, if you try to declare an instance of the Vector class without specifying 1 arg, you will get a compilation error:

  Vector v;  /// gives compilation error 

However, now consider the following class VectorContainer, which wraps the Vector class:

class VectorContainer
{
  Vector v;	/// OK - no compilation error !
}

Can anyone explain why this happens?


Thanks

[Moschops]

able to declare an instance

What instance?

You haven't yet tried to use the non-existent constructor. The error will occur when you try to actually create an instance of VectorContainer.

[SSteven]

Thanks for the reply.

I'm aware that the problem would occur when the ctor is included in VectorContainer.

My question really was why the problem doesn't occur when the ctor is not included. Hence I used the term "declaration" rather than "definition" of the var in question.

Thanks.

[tipaye]

class Vector
{
public:
    Vector(int s) {}
};

class VectorContainer
{
    Vector v; // -> this is a declaration just saying that 
              // -> VectorContainer has a member named v, of type Vector
              // -> there is no object creation or storage allocation
              // -> this is essentially a description 
              // -> this is what @moschops pointed out
};

int main()
{
    Vector v(3); // -> this is both a declaration and definition
                 // -> this means an object will be created
                 // -> a region of memory, the right size and type  will be
                 // -> this memory will be referred to as v in this scope
                 // -> to create this object the constructor Vector(int) is invoked
              
    Vector v1;   // -> just like above, except there's no constructor Vector()
                 // -> so compilation fails, as we have tried to call a function 
                 // -> that does not exist

    extern Vector v2;    // -> declaration only, no storage is allocated no constructor is called
                                  // -> compilation succeeds, v2 must be properly defined elsewhere 
                                  // -> before use

    VectorContainer v3;    // -> fails for same reason as line 24
                                      // -> when it tries to initialise its v member.

    return (0);
}

@SSteven it appears you've been caught out by the fact that if the compiler can figure it out, it will make a definition not a declaration

[Moschops]

My question really was why the problem doesn't occur when the ctor is not included.

Why would it? What did you ask the compiler to do that it doesn't know how to do? What function did the linker go looking for that it couldn't find?

The problem will occur when you try to use a constructor that doesn't exist. Your code doesn't try to use a constructor that doesn't exist, so there is no problem. That's why the problem does not occur. Because code not existing is only a problem when you try to use the code that doesn't exist.

You can declare functions (such as constructors) that don't exist all day long without any trouble. The problem only happens when you try to use (i.e. execute) functions that don't exist.