C++: Key Points - Constructors

Purpose

Constructors have one purpose - to allow you to guarantee that objects are correctly initialized. The compiler will guarantee that your constructors are called, inserting constructor calls as necessary. This is a great leap in reliability over C programming where, if the programmer forgot to initialize something explicitly, it contained whatever garbage was in memory.

Three types - No-parameter (default), copy constructors, and all others. Only the first two have unexpected aspects.

1. No-parameter (default) constructors

Default: If you don't declare any constructors, the compiler creates a default, parameterless constructor. Unfortunately, it does nothing good; it only satisfies the compiler's need for a parameterless constructor.
No default: If you declare constructors with parameters, no default parameterless constructor will be created.
Forbid creation. If you want control over creation of objects from you class (eg, to enforce use of a factory method, declare a private parameterless constructor.

Automatically called: The parameterless constructor for class A would be called in these circumstances:

    A x;              // Calls default constructor
    A xa = new A[99]; // Calls default constructor for each array element.

2. Copy constructors

Purpose: To initialize one object from another. This is different than assignment because there is no pre-existing value that may have to be destroyed.
Syntax:
```
    A(const A& a2) {
        . . .
    }
```
Note that the parameter must be a const reference.
Default creation. If you don't define a copy constructor, the compiler creates one which simply does a shallow memberwise copy. If you dynamically allocate memory for your class, you can only get a deep copy by writing your own copy constructor.

Automatically called: A copy constructor is called in the following circumstances.

A x(y);  // Where y is of type A.
f(x);    // A copy constructor is called for value parameters.
x = g(); // A copy constructor is called for value returns.

Preventing copying. To prevent copies, eg, to prevent an object from being passed by value, declare the copy constructor as private.

Questions

Assume A is a class.

Function definitions

A f(A x)   { return x;}
A g(A& x)  { return x;}
A& h(A& x) { return x;}

Questions

For each of the following function calls, how many times is the copy constructor called?

A a;
A b;

a = f(b);
a = g(b);
a = h(b);