Pointers to classes
Objects can also be pointed to by pointers: Once declared, a class becomes a valid type, so it can be used as the type pointed to by a pointer. For example:
is a pointer to an object of class
Rectangle.
Similarly as with plain data structures, the members of an object can be accessed directly from a pointer by using the arrow operator (
->). Here is an example with some possible combinations:
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// pointer to classes example
#include <iostream>
using namespace std;
class Rectangle {
int width, height;
public:
Rectangle(int x, int y) : width(x), height(y) {}
int area(void) { return width * height; }
};
int main() {
Rectangle obj (3, 4);
Rectangle * foo, * bar, * baz;
foo = &obj;
bar = new Rectangle (5, 6);
baz = new Rectangle[2] { {2,5}, {3,6} };
cout << "obj‘s area: " << obj.area() << ‘\n‘;
cout << "*foo‘s area: " << foo->area() << ‘\n‘;
cout << "*bar‘s area: " << bar->area() << ‘\n‘;
cout << "baz[0]‘s area:" << baz[0].area() << ‘\n‘;
cout << "baz[1]‘s area:" << baz[1].area() << ‘\n‘;
delete bar;
delete[] baz;
return 0;
}
|
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This example makes use of several operators to operate on objects and pointers (operators
*,
&,
.,
->,
[]). They can be interpreted as:
| expression |
can be read as |
*x |
pointed to by x |
&x |
address of x |
x.y |
member y of object x |
x->y |
member y of object pointed to by x |
(*x).y |
member y of object pointed to by x (equivalent to the previous one) |
x[0] |
first object pointed to by x |
x[1] |
second object pointed to by x |
x[n] |
(n+1)th object pointed to by x |
Most of these expressions have been introduced in earlier chapters. Most notably, the chapter about arrays introduced the offset operator (
[]) and the chapter about plain data structures introduced the arrow operator (
->).
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