FunctionPointers.md

Function Pointers in C++

A way to assign a function to a variable.

You can pass functions as parameters for other functions, as a consequence of the statement above.

#include <iostream>

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

int main()
{
    auto function = HelloWorld;
    // This is the type of our function pointer, (that `auto` is replacing)
    // void(*function)() = HelloWorld;

    function();
    function();
}

In the example above, we are getting the address of that HelloWorld function, note that we just need to remove the () to make our intent clear, that we don't want to execute (or get the output of) the function.

//Equivalent assignment, but more explicit.
// We don't need to use thee & because there is an implicit conversion.
auto function = &HelloWorld;

Since the type name of these function pointers are a little bit cumbersome, normally we can use auto or typedef'en into a more "friednly" type, basic creating an alias.

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

int main()
{
    typedef void(*HelloWorldFunction)();

    HelloWorldFunction function = HelloWorld;

    function();
}

Now using parameters

void HelloWorld(int a)
{
    std::cout << "Hello World! Value: " << a << std::endl;
}

int main()
{
    typedef void(*HelloWorldFunction)(int);

    HelloWorldFunction function = HelloWorld;

    function(8);
}

An useful example

#include <iostream>
#include <vector>

void PrintValue(int value)
{
    std::cout << "Value: " << value << std::endl;
}

void ForEach(const std::vector<int>& values, void(*func)(int))
{
    for (int value : values)
    {
        func(value);
    }
}

int main()
{
    std::vector<int> values = { 1, 5, 4, 2, 3};
    ForEach(values, PrintValue);

    // Or using a lambda
    ForEach(values, [](int value) { std::cout << "Value: " << std::endl; });
}

Extra material

https://stackoverflow.com/questions/4295432/typedef-function-pointer