Constructor overloading in C++ - Step-by-Step Execution

Concept Flow - Constructor overloading

Create object

↓

Call constructor

↓

Match constructor signature

↓

Constructor 1: no params

↓

Constructor 2: 1 param

↓

Constructor 3: 2 params

↓

Initialize object with matched constructor

↓

Object ready to use

When creating an object, the program chooses the constructor that matches the given parameters to initialize the object.

Execution Sample

C++

class Box {
public:
  int length;
  int width;
  Box() { length = 1; }
  Box(int l) { length = l; }
  Box(int l, int w) { length = l; width = w; }
};
Box b1; Box b2(5); Box b3(3,4);

This code creates three Box objects using different constructors based on the number of arguments.

Execution Table

Step	Object Created	Constructor Called	Parameters	State After Construction
1	b1	Box()	none	length=1, width=uninitialized
2	b2	Box(int l)	5	length=5, width=uninitialized
3	b3	Box(int l, int w)	3, 4	length=3, width=4
4	End	No more objects	N/A	All objects initialized

💡 All objects created with matching constructors, execution ends.

Variable Tracker

Variable	Start	After b1	After b2	After b3
b1.length	uninitialized	1	1	1
b1.width	uninitialized	uninitialized	uninitialized	uninitialized
b2.length	uninitialized	uninitialized	5	5
b2.width	uninitialized	uninitialized	uninitialized	uninitialized
b3.length	uninitialized	uninitialized	uninitialized	3
b3.width	uninitialized	uninitialized	uninitialized	4

Key Moments - 3 Insights

Why does b1 call the constructor with no parameters?

Because b1 is created without any arguments, so the program matches it to the constructor with no parameters as shown in step 1 of the execution_table.

What happens if we create an object with two parameters?

The constructor with two parameters is called, initializing both length and width, as seen in step 3 of the execution_table.

Why is width uninitialized for b1 and b2?

Because the constructors called for b1 and b2 do not set width, so it remains uninitialized as shown in the variable_tracker.

Visual Quiz - 3 Questions

Look at the execution_table, which constructor is called when creating b2?

A Box(int l) with one parameter

B Box() with no parameters

C Box(int l, int w) with two parameters

D No constructor is called

💡 Hint

Check step 2 in the execution_table where b2 is created.

At which step does the object have both length and width initialized?

A Step 1

B Step 2

C Step 3

D No step initializes both

💡 Hint

Look at the 'State After Construction' column in the execution_table.

If we create Box b4(7), what will be the value of b4.length according to variable_tracker logic?

A 1

B 7

C uninitialized

D 4

💡 Hint

Refer to how b2 is initialized with one parameter in variable_tracker.

Concept Snapshot

Constructor Overloading in C++:
- Define multiple constructors with different parameters.
- Object creation calls constructor matching given arguments.
- Allows flexible object initialization.
- Unused members remain uninitialized unless set.
- Helps create objects with different starting states.

Full Transcript

Constructor overloading means having several constructors in a class, each with different parameters. When you create an object, the program picks the constructor that matches the number and type of arguments you provide. For example, if you create an object without arguments, the constructor with no parameters runs. If you provide one or two arguments, the matching constructor runs. This lets you create objects with different initial values easily. Variables not set by a constructor remain uninitialized. This visual trace shows three objects created with different constructors and how their variables change.