Pointers to functions in C++

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Just as an array name is a constant pointer to the first element of the array, a function name is a constant pointer to the function. It is possible to declare a pointer variable that points to a function, and to invoke the function by using that pointer. This can be very useful; it allows you to create programs that decide which functions to invoke based on user input.

The only tricky part about function pointers is understanding the type of the object being pointed to. A pointer to int points to an integer variable, and a pointer to a function must point to a function of the appropriate return type and signature.

In the declaration

long (* funcPtr) (int);

funcPtr is declared to be a pointer (note the * in front of the name) that points to a function that takes an integer parameter and returns a long. The parentheses around * funcPtr are necessary because the parentheses around int bind more tightly, that is they have higher precedence than the indirection operator (*). Without the first parentheses this would declare a function that takes an integer and returns a pointer to a long. (Remember that spaces are meaningless here.)

Examine these two declarations:

long * Function (int);
long (* funcPtr) (int);

The first, Function (), is a function taking an integer and returning a pointer to a variable of type long. The second, funcPtr, is a pointer to a function taking an integer and returning a variable of type long.

The declaration of a function pointer will always include the return type and the parentheses indicating the type of the parameters, if any. Listing below illustrates the declaration and use of function pointers.

Pointers to functions.

1:     //Using function pointers
3:     #include <iostream.h>
5:     void Square (int&,int&);
6:     void Cube (int&, int&);
7:     void Swap (int&, int &);
8:     void GetVals(int&, int&);
9:     void PrintVals(int, int);
10:    enum BOOL { FALSE, TRUE };
12:    int main()
13:    {
14:       void (* pFunc) (int &, int &);
15:       BOOL fQuit = FALSE;
17:       int valOne=1, valTwo=2;
18:       int choice;
19:       while (fQuit == FALSE)
20:       {
21:          cout << "(0)Quit (1)Change Values (2)Square (3)Cube (4)Swap: ";
22:          cin >> choice;
23:          switch (choice)
24:          {
25:             case 1: pFunc = GetVals; break;
26:             case 2: pFunc = Square; break;
27:             case 3: pFunc = Cube; break;
28:             case 4: pFunc = Swap; break;
29:             default : fQuit = TRUE; break;
30:          }
32:          if (fQuit)
33:             break;
35:          PrintVals(valOne, valTwo);
36:          pFunc(valOne, valTwo);
37:          PrintVals(valOne, valTwo);
38:       }
39:     return 0;
40:    }
42:    void PrintVals(int x, int y)
43:    {
44:       cout << "x: " << x << " y: " << y << endl;
45:    }
47:    void Square (int & rX, int & rY)
48:    {
49:       rX *= rX;
50:       rY *= rY;
51:    }
53:    void Cube (int & rX, int & rY)
54:    {
55:       int tmp;
57:       tmp = rX;
58:       rX *= rX;
59:       rX = rX * tmp;
61:       tmp = rY;
62:       rY *= rY;
63:       rY = rY * tmp;
64:    }
66:    void Swap(int & rX, int & rY)
67:    {
68:       int temp;
69:       temp = rX;
70:       rX = rY;
71:       rY = temp;
72:    }
74:    void GetVals (int & rValOne, int & rValTwo)
75:    {
76:       cout << "New value for ValOne: ";
77:       cin >> rValOne;
78:       cout << "New value for ValTwo: ";
79:       cin >> rValTwo;
80: }

Output: (0)Quit (1)Change Values (2)Square (3)Cube (4)Swap: 1
x: 1 y: 2
New value for ValOne: 2
New value for ValTwo: 3
x: 2 y: 3
(0)Quit (1)Change Values (2)Square (3)Cube (4)Swap: 3
x: 2 y: 3
x: 8 y: 27
(0)Quit (1)Change Values (2)Square (3)Cube (4)Swap: 2
x: 8 y: 27
x: 64 y: 729
(0)Quit (1)Change Values (2)Square (3)Cube (4)Swap: 4
x: 64 y: 729
x: 729 y: 64
(0)Quit (1)Change Values (2)Square (3)Cube (4)Swap: 0

Analysis: On lines 5-8, four functions are declared, each with the same return type and signature, returning void and taking two references to integers.

On line 14, pFunc is declared to be a pointer to a function that returns void and takes two integer reference parameters. Any of the previous functions can be pointed to by pFunc. The user is repeatedly offered the choice of which functions to invoke, and pFunc is assigned accordingly. On lines 35-36, the current value of the two integers is printed, the currently assigned function is invoked, and then the values are printed again.

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