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8.3. Accessing instance variables

You can read the values of an instance variable using the same syntax we used to write them:

int x = blank.x;

The expression blank.x means “go to the object named blank and get the value of x.” In this case we assign that value to a local variable named x. Notice that there is no conflict between the local variable named x and the instance variable named x. The purpose of dot notation is to identify which variable you are referring to unambiguously.

You can use dot notation as part of any C++ expression, so the following are legal.

cout << blank.x << ", " << blank.y << endl;
double distance = sqrt(blank.x * blank.x + blank.y * blank.y);

In the active code below, we access the instance variables of Point object black using dot notation and output their values. Next, we output the distance from the origin.

#include <iostream> #include <cmath> using namespace std; struct Point { double x, y; }; int main() { Point blank; blank.x = 3.0; blank.y = 4.0; cout << blank.x << ", " << blank.y << endl; double distance = sqrt(blank.x * blank.x + blank.y * blank.y); cout << distance << endl; }

Q-2: In string x = thing.cube;, what is the object and what is the instance variable we are reading the value of?

• string is the instance variable, cube is the object

• string is a data type.

• x is the instance variable, thing is the object

• x is the local variable.

• thing is the instance variable, cube is the object

• Consider the placement of thing – it is before the .

• cube is the instance variable, thing is the object

• Yes, we access the instance variable cube of the object thing using the dot operator.

• cube is the instance variable, string is the object

• string is a data type.

Q-3: What will print?

struct Blue {
  double x, y;
};

int main() {
  Blue blank;
  blank.x = 7.0;
  blank.y = 2.0;
  cout << blank.y << blank.x;
  double distance = blank.x * blank.x + blank.y * blank.y;
  cout << distance << endl;
}

• 2.0 7.0 53

• Spaces need to be printed out like any other output.

• 2753

• There are no spaces in the correct output.

• 7253

• The order in which the variables are printed out do not need to match the order in which they are declared.

• 7.02.053

• The order in which the variables are printed out do not need to match the order in which they are declared.

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Q-4: You want to go to the object named circle and get the integer value of y, then assign it to the local variable x. How would you do that?

• int y = circle.x();

• No parentheses are needed.

• int circle = x.y;

• You should be assigning to the local variable x.

• int y = circle.x;

• You should be assigning to the local variable x.

• int x = circle.y;

• This is the correct way to assign the value of y to x.

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