Thread: Implicit Constructor

I'm having trouble understanding these few sentances from a book that i'm reading. The sentace referers to this code:

Code:

// englobj.cpp
// objects using English measurements
#include <iostream.h>

class Distance                    // English Distance class
   {
   private:
      int feet;
      float inches;
   public:
      void setdist(int ft, float in)   // set distance to args
	 { feet = ft; inches = in; }

      void getdist()              // get length from user
	 {
	 cout << "\nEnter feet: ";  cin >> feet;
	 cout << "Enter inches: ";  cin >> inches;
	 }

      void showdist()             // display distance
	 { cout << feet << "\'-" << inches << '\"'; }
   };

void main()
   {
   Distance dist1, dist2;         // define two lengths

   dist1.setdist(11, 6.25);       // set dist1

   dist2.getdist();               // get dist2 from user

				  // display lengths
   cout << "\ndist1 = ";  dist1.showdist();
   cout << "\ndist2 = ";  dist2.showdist();
   }

"...However, we also want to define variables of type DISTANCE without initializing them, as we did in ENGLOBG.
Distance dist1, dist2;
In that program there was no constructor, but our definition worked just fine. How could they work without a constructor? Because an implicit constructor was built into the program automatically by the compiler and it created the objects, even thought we didn't define it in the class..."

Can someone explain what hes' talking about?

Every class has a constructor. If u dont specify it the compiler will automatically create one for you.

BY THE WAY. is this that Robert Lafore book object oriented programming with c++

ya it is, it's his second edition OOP. how'd you know?
another quesiton. why does it need a constructor anyways?

When you declare a class, the compiler supplies a default constructor automatically, but only as long as you don't define a constructor for the class yourself. A constructor is called when a new instance of a class is defined. It provides the opportunity to initialize the new object as it's created. The default constructor that's generated by the compiler does nothing--in particular, the default constructor does not initialize the data members of the object that's created, and therefore they will contain junk values.

"why does it need a constructor anyways?"

To create the object.

You can always tell which function in a class is the constructor because it has the same name as the class and no return type.

so does every data type in C++ need a constructor to create it. for example does
int, float, double, long, struct types, classes
all need a constructor supplied by the comiiler atutomatically to create them?

so does this mean you can add your own constructor to different data constructs, such as structures? and i don't understand why you need a constructor left blank ( cool { } ) when you declare your own constructors. Ive seen some class constructs that declare constructors but do not include cool { } (empty one) while others declare their own constructors but include the cool { }(empty one)

Quoting one of the greats:

"I recognize the words, but they don't make any sense."

[edit]
AhhhhHAH! Now, I think I understand. Reread this statement:

"When you declare a class, the compiler supplies a default constructor automatically, but only as long as you don't define a constructor for the class yourself."

Ok, so now we are getting somewhere. What if I define a constructor like this:

Code:

cool(int a, int b)
{
    data_member1=a;
    data_member2=b;
}

Based on the statement above, now the compiler WON'T supply a default constructor. So, if I do this:

cool a(3, 5);
cool b;

The second line will cause an error, because there is no constructor defined that takes no parameters. I didn't declare a default constructor, and the compiler doesn't supply one automatically if you define any constructor yourself, so there isn't a default constructor and the second line results in an-->>>ERROR.

You can define many different constructors for your class, but if you define even one, then there won't be a default constructor, so if you want to create objects with a default constructor, then you have to define it yourself:

cool ()
{ }

If you don't want to create objects without initializing them in a constructor(i.e you don't want to create a default constructor), then don't define one.

hmmm..ok 7stud. thanks, you clarified it for me. yah my question was strange now that i read it again

but coolio . thanks

Is there any advantages, or reasons in declaring a function inside a class but defining it some place else in the listing. The only reason i see is defining it closer to the function call makes it easier for the programmer to read teh code.