Function Pointers

A pointer to a function can be used to:

• Invoke the function it is pointing to.
• Pass the function it is pointing to, to another function as a parameter.

Getting Address

To return address of foo(), we can use either foo or &foo.
Both are valid.

Calling foo()

Assuming ptr points to foo(), we can invoke foo() by

• ptr()
• (*ptr)()

Both are valid.

void foo () {
    std::cout << "Hello World!" << std::endl;
}

int main () {
    // &foo or foo, both return foo's address
    void (*ptr_1)() = foo;
    void (*ptr_2)() = &foo;

    // both of the following are valid to call foo.
    ptr_1();
    (*ptr_2)();
}

Output:

Hello World!
Hello World!

foo() with parameters

Assume that the function takes 2 int parameters and returns a char, i.e. char foo(int a, int b).
In this case, the function pointer will look like char (*ptr)(int, int) = foo

char foo(int a, int b) {
    char c = a + b;
    return c;
}

int main () {
    char (*ptr)(int, int) = foo;
    std::cout << ptr(65, 1) << std::endl;
}

Output:

B

Passing Functions as Parameters

int foo(int a, int b) {
    return a + b;
}

void sum(int (*fuzz)(int, int), int x, int y) {
    std::cout << fuzz(x, y) << std::endl;
}

int main () {
    sum(foo, 4, 5);
}

Output

9

Array of Functions

Using the [] in the pointer declaration, we can have an array of functions.

int sum(int a, int b) {
    return a + b;
}

int sub(int a, int b) {
    return a - b;
}

int mul(int a, int b) {
    return a * b;
}

int main () {
    int (*ptr[])(int, int) = {sum, sub, mul};

    std::cout << ptr[0](5, 4) << std::endl;  // sum
    std::cout << ptr[1](5, 4) << std::endl;  // sub
    std::cout << ptr[2](5, 4) << std::endl;  // mul
}