When and How Should I Use Callback Functions in C ?

Callback Functions in C

When and How to Use Callback Functions:

Callback functions are function pointers or objects that are passed as arguments to another function. These callback functions are then invoked within the called function to customize its behavior based on the provided callback logic.

Common scenarios where callback functions are used:

• Writing generic code that can work with different logic by accepting different callbacks.
• Implementing event-based systems to notify interested parties of specific events.
• Enabling dynamic runtime behavior, such as changing the behavior of a certain action based on user input.

Types of Callables in C (11):

Callback functionality can be implemented using several types of callables, including:

• Function pointers (including pointers to member functions)
• std::function objects
• Lambda expressions
• Bind expressions
• Function objects (classes with overloaded function call operator operator())

Writing Callbacks:

The syntax for writing callback types differs depending on the type of callable used. Here is an overview:

1. Function Pointers:

typedef int (*f_int_t)(int);
int (* foo_p)(int);

2. Pointers to Member Functions:

typedef int (C::* f_C_int_t)(int);
int (C::* C_foo_p)(int);

3. std::Function:

std::function<int(int)> stdf_foo = &amp;foo;
std::function<int(const C&, int)> stdf_C_foo = &amp;C::foo;

4. Templated Callback Type:

template<class R, class T>
void stdf_transform_every_int_templ(int * v, unsigned const n, std::function<R(T)> fp)

Calling Callbacks:

The syntax for calling callbacks varies depending on the type of callable. Here is an overview:

1. Function Pointers:

int b = foobar(a, foo);

2. Pointers to Member Functions:

int b = C_foobar(a, my_c, &C::foo);

3. std::Function:

int b = stdf_foobar(a, stdf_foo);

4. Templated Callback Type:

stdf_transform_every_int_templ(&a[0], 5, &foo);

Example:

Consider the following example of a function that transforms every integer in an array using a callback:

void tranform_every_int(int * v, unsigned n, int (*fp)(int))
{
  for (unsigned i = 0; i < n; ++i)
  {
    v[i] = fp(v[i]);
  }
}

This function can be used with different callback functions to achieve different behavior:

// Using a function pointer
int double_int(int x) { return 2*x; }
int a[5] = {1, 2, 3, 4, 5};
tranform_every_int(&a[0], 5, double_int); // Transform the array using the double_int callback

// Using lambda expression
tranform_every_int(&a[0], 5, [](int x) -> int { return x/2; }); // Transform the array using an anonymous function that divides each element by 2

// Using std::bind
int nine_x_and_y (int x, int y) { return 9*x + y; }
std::placeholders::_1;
tranform_every_int(&a[0], 5, std::bind(nine_x_and_y, _1, 4)); // Transform the array using std::bind with nine_x_and_y

In conclusion, callback functions provide a powerful and flexible mechanism for customizing the behavior of functions and enabling dynamic runtime behavior in C . They can be implemented using various callables, and their syntax varies accordingly.

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