11.7. Constructors¶
Another function we wrote in Section 9.1 was
makeTime:
Time makeTime (double secs) {
Time time;
time.hour = int (secs / 3600.0);
secs -= time.hour * 3600.0;
time.minute = int (secs / 60.0);
secs -= time.minute * 60.0;
time.second = secs;
return time;
}
Of course, for every new type, we need to be able to create new objects.
In fact, functions like makeTime are so common that there is a
special function syntax for them. These functions are called
constructors and the syntax looks like this:
Time::Time (double secs) {
hour = int (secs / 3600.0);
secs -= hour * 3600.0;
minute = int (secs / 60.0);
secs -= minute * 60.0;
second = secs;
}
First, notice that the constructor has the same name as the class, and no return type. The arguments haven’t changed, though.
Second, notice that we don’t have to create a new time object, and we
don’t have to return anything. Both of these steps are handled
automatically. We can refer to the new object—the one we are
constructing—using the keyword this, or implicitly as shown here.
When we write values to hour, minute and second, the
compiler knows we are referring to the instance variables of the new
object.
To invoke the constructor, you use syntax that is a cross between a variable declaration and a function call:
Time time (seconds);
This statement declares that the variable time has type Time,
and it invokes the constructor we just wrote, passing the value of
seconds as an argument. The system allocates space for the new
object and the constructor initializes its instance variables. The
result is assigned to the variable time.
- They initialize the instance variables of an object.
- Incorrect! This statement is true!
- They have the same name as the class.
- Incorrect! This statement is true!
- They return an instance of an object.
- Correct! Constructors do not have a return type.
- We refer to the objects they initialize implicitly, or using keyword this.
- Incorrect! This statment is true!
- They have the same name as the class, no return type and unchanged parameters.
- Incorrect! This statement is true!
Q-2: Which statment is false about constructors?
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